Bug 1289976. Part 5 - Remove the IsShutdown() check from MediaDecoder::Pause(). r=kaku

1. Pause() is called from HTMLMediaElement and happens before Shutdown().
2. Pause() is called from SetPlaybackRate() which is called from HTMLMediaElement.

MozReview-Commit-ID: DDr7Bg8jkF2

--HG--
extra : rebase_source : 8b8a4fdc914d0bce7572d9a1ccb0f530bdba062c

dom/media/MediaDecoder.cpp

@@ -421,9 +421,7 @@ void
 MediaDecoder::Pause()
 {
   MOZ_ASSERT(NS_IsMainThread());
-  if (IsShutdown()) {
-    return;
-  }
+  MOZ_ASSERT(!IsShutdown());
   if (mPlayState == PLAY_STATE_LOADING || IsEnded()) {
     mNextState = PLAY_STATE_PAUSED;
     return;