[Fuchsia] use timeline presence as indicator that playback has started.

Previously FuchsiaAudioRenderer was expecting timeline update with
stream_delta > 0 after Start(). That doesn't work correctly when
playback is started with playbackRate=0.0 . Presence of
presentation_timeline attribute in AudioConsumerStatus indicates that
playback has started, i.e. Start() call has been handled, so it's not
necessary to check stream_delta > 0.

Bug: 1027048
Change-Id: Ie4de38c34a2ce6cb57faf567b784275b3a2d5a25
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1976285
Commit-Queue: Sergey Ulanov <sergeyu@chromium.org>
Reviewed-by: Kevin Marshall <kmarshall@chromium.org>
Auto-Submit: Sergey Ulanov <sergeyu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#726492}

media/fuchsia/audio/fuchsia_audio_renderer.cc

@@ -347,10 +347,8 @@ void FuchsiaAudioRenderer::OnAudioConsumerStatusChanged(
   }
 
   if (status.has_presentation_timeline()) {
-    bool playing = status.presentation_timeline().subject_delta > 0;
-
     base::AutoLock lock(state_lock_);
-    if (playing && state_ != PlaybackState::kStopped) {
+    if (state_ != PlaybackState::kStopped) {
       if (state_ == PlaybackState::kStarting) {
         state_ = PlaybackState::kPlaying;
       }