[GSTMediaPlayer] Make fake preroll asynchronous (#1232)

1) Call didPreroll() also for <video> elements that are audio-only
2) Make didPreroll() call async and put in onto HTML media element
   task queue to make sure it is executed after dispatching
   'seeking' event to JS so app has a chance to note HTMLmedia.seeking
   attribute is 'true'

Author: asurdej-comcast

Source/WebCore/platform/graphics/gstreamer/mse/MediaPlayerPrivateGStreamerMSE.cpp

@@ -262,13 +262,22 @@ bool MediaPlayerPrivateGStreamerMSE::doSeek(const MediaTime& position, float rat
 
     m_mediaSource->seekToTime(m_seekTime);
 
-    if (m_player && !m_player->isVideoPlayer() && m_audioSink) {
+    if (m_player && !hasVideo() && m_audioSink) {
         gboolean audioSinkPerformsAsyncStateChanges;
         g_object_get(m_audioSink.get(), "async", &audioSinkPerformsAsyncStateChanges, nullptr);
         if (!audioSinkPerformsAsyncStateChanges) {
             // If audio-only pipeline's sink is not performing async state changes
             // we must simulate preroll right away as otherwise nothing will trigger it.
-            didPreroll();
+
+            // Post this on HTML media element queue so it will be executed
+            // synchonously with media events (e.g. seeking). This will ensure
+            // that HTML element attributes (like HTMLmedia.seeking) are not reseted
+            // before app receives "seeking" event
+            m_player->queueTaskOnEventLoop([weakThis = WeakPtr { *this }, this] {
+                if (!weakThis)
+                    return;
+                didPreroll();
+            });
         }
     }