Bug 1144509: Part1. Fire timeupdate before seeked event as per spec. r=karlt

dom/html/HTMLMediaElement.cpp

@@ -3243,6 +3243,7 @@ void HTMLMediaElement::SeekCompleted()
 {
   mPlayingBeforeSeek = false;
   SetPlayedOrSeeked(true);
+  FireTimeUpdate(false);
   DispatchAsyncEvent(NS_LITERAL_STRING("seeked"));
   // We changed whether we're seeking so we need to AddRemoveSelfReference
   AddRemoveSelfReference();