[Fuchsia] Fix FuchsiaAudioRenderer::StopTicking() to set correct state

After crrev.com/738096 StopTicking() was setting state to kStarting
instead of kStoppped. Fix it.

Change-Id: Ic1fab2c0cbfcc6636e20e9adb3bd536065dbf843
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2051389Reviewed-by: David Dorwin <ddorwin@chromium.org>
Commit-Queue: Sergey Ulanov <sergeyu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#740846}

media/fuchsia/audio/fuchsia_audio_renderer.cc

@@ -266,7 +266,7 @@ void FuchsiaAudioRenderer::StopTicking() {
   audio_consumer_->Stop();
 
   base::AutoLock lock(timeline_lock_);
-  SetPlaybackState(PlaybackState::kStarting);
+  SetPlaybackState(PlaybackState::kStopped);
   media_pos_ = CurrentMediaTimeLocked();
 }