Bug 1457960 - P1. make MediaDecoder::Seek returns void. r=bryce

MozReview-Commit-ID: 2pbZprnYqcF

dom/html/HTMLMediaElement.cpp

@@ -2850,11 +2850,7 @@ HTMLMediaElement::Seek(double aTime,
   // The media backend is responsible for dispatching the timeupdate
   // event if it changes the playback position as a result of the seek.
   LOG(LogLevel::Debug, ("%p SetCurrentTime(%f) starting seek", this, aTime));
-  nsresult rv = mDecoder->Seek(aTime, aSeekType);
-  if (NS_FAILED(rv)) {
-    aRv.Throw(rv);
-    return nullptr;
-  }
+  mDecoder->Seek(aTime, aSeekType);
 
   // We changed whether we're seeking so we need to AddRemoveSelfReference.
   AddRemoveSelfReference();