Bug 1289976. Part 6 - Remove the IsShutdown() check from MediaDecoder::RemoveMediaTracks(). r=kaku

1. It is called from ChangeState() when IsEnded() is true.
2. It is called from OnMetadataUpdate(). The callback is disconnected in Shutdown().

MozReview-Commit-ID: 8m4jtcl91hT

--HG--
extra : rebase_source : 0988128ec6d05f07ab7b072cc70a9c2970c829c5

dom/media/MediaDecoder.cpp

@@ -1892,10 +1892,7 @@ void
 MediaDecoder::RemoveMediaTracks()
 {
   MOZ_ASSERT(NS_IsMainThread());
-
-  if (IsShutdown()) {
-    return;
-  }
+  MOZ_ASSERT(!IsShutdown());
 
   HTMLMediaElement* element = mOwner->GetMediaElement();
   if (!element) {