LibDevTools: Implement enough of the protocol to inspect tabs

There is a lot needed all at once to actually inspect a tab's DOM tree.
It begins with requesting a "watcher" from a TabActor. It seems there
can be many types of watchers, but here we implement the "frame" watcher
only. The watcher creates an "inspector", which in turn creates a
"walker", which is the actor ultimately responsible for serializing and
inspecting the DOM tree.

In between all that, the DevTools client will send a handful of other
informational requests. If we do not reply to these, the client will not
move forward with the walker. For example, the CSSPropertiesActor will
be asked for a list of all known CSS properties.

Libraries/LibDevTools/Actors/WatcherActor.cpp

@@ -0,0 +1,140 @@
+/*
+ * Copyright (c) 2025, Tim Flynn <trflynn89@ladybird.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/JsonObject.h>
+#include <LibCore/EventLoop.h>
+#include <LibDevTools/Actors/CSSPropertiesActor.h>
+#include <LibDevTools/Actors/FrameActor.h>
+#include <LibDevTools/Actors/InspectorActor.h>
+#include <LibDevTools/Actors/TabActor.h>
+#include <LibDevTools/Actors/TargetConfigurationActor.h>
+#include <LibDevTools/Actors/ThreadActor.h>
+#include <LibDevTools/Actors/ThreadConfigurationActor.h>
+#include <LibDevTools/Actors/WatcherActor.h>
+#include <LibDevTools/DevToolsServer.h>
+
+namespace DevTools {
+
+NonnullRefPtr<WatcherActor> WatcherActor::create(DevToolsServer& devtools, ByteString name, WeakPtr<TabActor> tab)
+{
+    return adopt_ref(*new WatcherActor(devtools, move(name), move(tab)));
+}
+
+WatcherActor::WatcherActor(DevToolsServer& devtools, ByteString name, WeakPtr<TabActor> tab)
+    : Actor(devtools, move(name))
+    , m_tab(move(tab))
+{
+}
+
+WatcherActor::~WatcherActor() = default;
+
+void WatcherActor::handle_message(StringView type, JsonObject const& message)
+{
+    JsonObject response;
+    response.set("from"sv, name());
+
+    if (type == "getParentBrowsingContextID"sv) {
+        auto browsing_context_id = message.get_integer<u64>("browsingContextID"sv);
+        if (!browsing_context_id.has_value()) {
+            send_missing_parameter_error("browsingContextID"sv);
+            return;
+        }
+
+        response.set("browsingContextID"sv, *browsing_context_id);
+        send_message(move(response));
+        return;
+    }
+
+    if (type == "getTargetConfigurationActor"sv) {
+        if (!m_target_configuration)
+            m_target_configuration = devtools().register_actor<TargetConfigurationActor>();
+
+        response.set("configuration"sv, m_target_configuration->serialize_configuration());
+        send_message(move(response));
+        return;
+    }
+
+    if (type == "getThreadConfigurationActor"sv) {
+        if (!m_thread_configuration)
+            m_thread_configuration = devtools().register_actor<ThreadConfigurationActor>();
+
+        response.set("configuration"sv, m_thread_configuration->serialize_configuration());
+        send_message(move(response));
+        return;
+    }
+
+    if (type == "watchTargets"sv) {
+        auto target_type = message.get_byte_string("targetType"sv);
+        if (!target_type.has_value()) {
+            send_missing_parameter_error("targetType"sv);
+            return;
+        }
+
+        if (target_type == "frame"sv) {
+            auto& css_properties = devtools().register_actor<CSSPropertiesActor>();
+            auto& inspector = devtools().register_actor<InspectorActor>(m_tab);
+            auto& thread = devtools().register_actor<ThreadActor>();
+
+            auto& target = devtools().register_actor<FrameActor>(m_tab, css_properties, inspector, thread);
+            m_target = target;
+
+            response.set("type"sv, "target-available-form"sv);
+            response.set("target"sv, target.serialize_target());
+            send_message(move(response));
+
+            target.send_frame_update_message();
+
+            JsonObject message;
+            message.set("from", name());
+            send_message(move(message));
+
+            return;
+        }
+    }
+
+    send_unrecognized_packet_type_error(type);
+}
+
+JsonObject WatcherActor::serialize_description() const
+{
+    JsonObject resources;
+    resources.set("Cache"sv, false);
+    resources.set("console-message"sv, false);
+    resources.set("cookies"sv, false);
+    resources.set("css-change"sv, false);
+    resources.set("css-message"sv, false);
+    resources.set("css-registered-properties"sv, false);
+    resources.set("document-event"sv, false);
+    resources.set("error-message"sv, false);
+    resources.set("extension-storage"sv, false);
+    resources.set("indexed-db"sv, false);
+    resources.set("jstracer-state"sv, false);
+    resources.set("jstracer-trace"sv, false);
+    resources.set("last-private-context-exit"sv, false);
+    resources.set("local-storage"sv, false);
+    resources.set("network-event"sv, false);
+    resources.set("network-event-stacktrace"sv, false);
+    resources.set("platform-message"sv, false);
+    resources.set("reflow"sv, false);
+    resources.set("server-sent-event"sv, false);
+    resources.set("session-storage"sv, false);
+    resources.set("source"sv, false);
+    resources.set("stylesheet"sv, false);
+    resources.set("thread-state"sv, false);
+    resources.set("websocket"sv, false);
+
+    JsonObject description;
+    description.set("shared_worker"sv, false);
+    description.set("service_worker"sv, false);
+    description.set("frame"sv, true);
+    description.set("process"sv, false);
+    description.set("worker"sv, false);
+    description.set("resources"sv, move(resources));
+
+    return description;
+}
+
+}