LibWeb: Do not spin the event loop awaiting text track state changes

The text track processing model would previously spin forever waiting
for the track URL to change. It would then recursively invoke itself
to handle the new URL, again entering the spin loop. This meant that
tracks could easily cause event loop hangs.

We now have an observer system to be notified when the track state
changes instead. This lets us exit the processing model and move on.

Libraries/LibWeb/HTML/TextTrackObserver.cpp

@@ -0,0 +1,43 @@
+/*
+ * Copyright (c) 2025, Tim Flynn <trflynn89@ladybird.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibJS/Runtime/Realm.h>
+#include <LibJS/Runtime/VM.h>
+#include <LibWeb/HTML/TextTrackObserver.h>
+
+namespace Web::HTML {
+
+GC_DEFINE_ALLOCATOR(TextTrackObserver);
+
+TextTrackObserver::TextTrackObserver(JS::Realm& realm, TextTrack& text_track)
+    : Bindings::PlatformObject(realm)
+    , m_text_track(text_track)
+{
+    m_text_track->register_observer({}, *this);
+}
+
+void TextTrackObserver::visit_edges(Cell::Visitor& visitor)
+{
+    Base::visit_edges(visitor);
+    visitor.visit(m_text_track);
+    visitor.visit(m_track_readiness_observer);
+}
+
+void TextTrackObserver::finalize()
+{
+    Base::finalize();
+    m_text_track->unregister_observer({}, *this);
+}
+
+void TextTrackObserver::set_track_readiness_observer(Function<void(TextTrack::ReadinessState)> callback)
+{
+    if (callback)
+        m_track_readiness_observer = GC::create_function(vm().heap(), move(callback));
+    else
+        m_track_readiness_observer = nullptr;
+}
+
+}