LibWeb: Change the callback used to detect the end of a video stream

The on_end_of_stream callback was added to notify clients that video
playback has stopped when we didn't have a way to retrieve the playback
state from Video::PlaybackManager. Now that we do, we should consolidate
on using the on_playback_state_change callback to detect such changes.

Userland/Libraries/LibWeb/HTML/VideoTrack.cpp

@@ -37,9 +37,17 @@ VideoTrack::VideoTrack(JS::Realm& realm, JS::NonnullGCPtr<HTMLMediaElement> medi
         m_media_element->set_current_playback_position(playback_position_ms / 1000.0);
     };
 
-    m_playback_manager->on_end_of_stream = [this]() {
-        auto playback_position_ms = static_cast<double>(duration().to_milliseconds());
-        m_media_element->set_current_playback_position(playback_position_ms / 1000.0);
+    m_playback_manager->on_playback_state_change = [this]() {
+        switch (m_playback_manager->get_state()) {
+        case Video::PlaybackState::Stopped: {
+            auto playback_position_ms = static_cast<double>(duration().to_milliseconds());
+            m_media_element->set_current_playback_position(playback_position_ms / 1000.0);
+            break;
+        }
+
+        default:
+            break;
+        }
     };
 
     m_playback_manager->on_decoder_error = [](auto) {