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/FrameActor.cpp

@@ -0,0 +1,98 @@
+/*
+ * Copyright (c) 2025, Tim Flynn <trflynn89@ladybird.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/JsonArray.h>
+#include <AK/JsonObject.h>
+#include <LibDevTools/Actors/CSSPropertiesActor.h>
+#include <LibDevTools/Actors/FrameActor.h>
+#include <LibDevTools/Actors/InspectorActor.h>
+#include <LibDevTools/Actors/TabActor.h>
+#include <LibDevTools/Actors/ThreadActor.h>
+
+namespace DevTools {
+
+NonnullRefPtr<FrameActor> FrameActor::create(DevToolsServer& devtools, ByteString name, WeakPtr<TabActor> tab, WeakPtr<CSSPropertiesActor> css_properties, WeakPtr<InspectorActor> inspector, WeakPtr<ThreadActor> thread)
+{
+    return adopt_ref(*new FrameActor(devtools, move(name), move(tab), move(css_properties), move(inspector), move(thread)));
+}
+
+FrameActor::FrameActor(DevToolsServer& devtools, ByteString name, WeakPtr<TabActor> tab, WeakPtr<CSSPropertiesActor> css_properties, WeakPtr<InspectorActor> inspector, WeakPtr<ThreadActor> thread)
+    : Actor(devtools, move(name))
+    , m_tab(move(tab))
+    , m_css_properties(move(css_properties))
+    , m_inspector(move(inspector))
+    , m_thread(move(thread))
+{
+}
+
+FrameActor::~FrameActor() = default;
+
+void FrameActor::handle_message(StringView type, JsonObject const&)
+{
+    JsonObject response;
+    response.set("from"sv, name());
+
+    if (type == "listFrames"sv) {
+        send_message(move(response));
+        return;
+    }
+
+    send_unrecognized_packet_type_error(type);
+}
+
+void FrameActor::send_frame_update_message()
+{
+    JsonArray frames;
+
+    if (auto tab_actor = m_tab.strong_ref()) {
+        JsonObject frame;
+        frame.set("id"sv, tab_actor->description().id);
+        frame.set("title"sv, tab_actor->description().title);
+        frame.set("url"sv, tab_actor->description().url);
+        frames.must_append(move(frame));
+    }
+
+    JsonObject message;
+    message.set("from", name());
+    message.set("type", "frameUpdate"sv);
+    message.set("frames", move(frames));
+    send_message(move(message));
+}
+
+JsonObject FrameActor::serialize_target() const
+{
+    JsonObject traits;
+    traits.set("frames"sv, true);
+    traits.set("isBrowsingContext"sv, true);
+    traits.set("logInPage"sv, false);
+    traits.set("navigation"sv, true);
+    traits.set("supportsTopLevelTargetFlag"sv, true);
+    traits.set("watchpoints"sv, true);
+
+    JsonObject target;
+    target.set("actor"sv, name());
+
+    if (auto tab_actor = m_tab.strong_ref()) {
+        target.set("title"sv, tab_actor->description().title);
+        target.set("url"sv, tab_actor->description().url);
+        target.set("browsingContextID"sv, tab_actor->description().id);
+        target.set("outerWindowID"sv, tab_actor->description().id);
+        target.set("isTopLevelTarget"sv, true);
+    }
+
+    target.set("traits"sv, move(traits));
+
+    if (auto css_properties = m_css_properties.strong_ref())
+        target.set("cssPropertiesActor"sv, css_properties->name());
+    if (auto inspector = m_inspector.strong_ref())
+        target.set("inspectorActor"sv, inspector->name());
+    if (auto thread = m_thread.strong_ref())
+        target.set("threadActor"sv, thread->name());
+
+    return target;
+}
+
+}