Bug 1580659 - part3 : call 'WindowAudioCaptureChanged()' explicitly. r=chunmin,karlt

Previously `AudioChannelService` called `WindowAudioCaptureChanged()` implicitly whenever we added the agent to the service [1], which made the audio capturing callback happen before finishing registration.

There are two drawbacks,
(1) it's hard to be aware of that the audio capturing callback would be called before finishing `AudioChannelAgent::NotifyStartedPlaying()` [2], which causes unclear call flow.

(2) If someone checks `AudioChannelAgent::IsPlayingStarted()` [3] inside audio capturing callback, then we would find that the `mIsRegToService` is false even if we have registered the agent to `AudioChannelService` because `mIsRegToService` is updated in the last line in the `AudioChannelAgent::NotifyStartedPlaying()`, but the audio capturing callback could be executed before that.

[1] https://searchfox.org/mozilla-central/rev/a777ff11b6d700a698c61e5bd17e73b044304494/dom/audiochannel/AudioChannelService.cpp#723
[2] https://searchfox.org/mozilla-central/rev/a777ff11b6d700a698c61e5bd17e73b044304494/dom/audiochannel/AudioChannelAgent.cpp#144
[3] https://searchfox.org/mozilla-central/rev/a777ff11b6d700a698c61e5bd17e73b044304494/dom/audiochannel/AudioChannelAgent.cpp#285

Differential Revision: https://phabricator.services.mozilla.com/D45750

dom/html/HTMLMediaElement.cpp

@@ -1198,6 +1198,7 @@ class HTMLMediaElement::AudioChannelAgentCallback final
     NotifyMediaStarted(mAudioChannelAgent->WindowID());
     WindowVolumeChanged(config.mVolume, config.mMuted);
     WindowSuspendChanged(config.mSuspend);
+    WindowAudioCaptureChanged(config.mCapturedAudio);
   }
 
   void StopAudioChanelAgent() {