Bug 1289976. Part 7 - Remove the IsShutdown() check from MediaDecoder::Seek(). r=kaku

1. It is called from DurationChanged() which returns early when IsShutdown() is true.
2. It is called from Play() when IsEnded() is true.

MozReview-Commit-ID: Ixy5OMZHxIm

--HG--
extra : rebase_source : 4cd7229084c6af8b7123ee6c85156ef4932308f3

dom/media/MediaDecoder.cpp

@@ -801,7 +801,7 @@ nsresult
 MediaDecoder::Seek(double aTime, SeekTarget::Type aSeekType, dom::Promise* aPromise /*=nullptr*/)
 {
   MOZ_ASSERT(NS_IsMainThread());
-  NS_ENSURE_TRUE(!IsShutdown(), NS_ERROR_FAILURE);
+  MOZ_ASSERT(!IsShutdown());
 
   UpdateDormantState(false /* aDormantTimeout */, true /* aActivity */);