Bug 1290780. Part 4 - Assert IsShutdown() is false in SeekingStarted() since the callback is disconnected in Shutdown(). r=kaku

MozReview-Commit-ID: 7Q9EAhy3O7w

--HG--
extra : rebase_source : a68a9d50d0c2c669dd4c5ae0897e06c1538811d1

dom/media/MediaDecoder.cpp

@@ -1306,9 +1306,7 @@ void
 MediaDecoder::SeekingStarted(MediaDecoderEventVisibility aEventVisibility)
 {
   MOZ_ASSERT(NS_IsMainThread());
-  if (IsShutdown())
-    return;
-
+  MOZ_ASSERT(!IsShutdown());
   if (aEventVisibility != MediaDecoderEventVisibility::Suppressed) {
     mOwner->SeekStarted();
   }