Bug 1347402 part 2 - open a GetOwnerDoc() interface at the MediaDecoderOwner; r=jwwang

Open a GetOwnerDoc() method to the MediaDecoderOwner interface and then we can get the
owner document via a pointer to MediaDecoderOwner in MediaDecoder.

MozReview-Commit-ID: JCzQDLx1MsU

dom/html/HTMLMediaElement.cpp

@@ -7410,6 +7410,12 @@ HTMLMediaElement::NotifyDecoderActivityChanges() const
   }
 }
 
+nsIDocument*
+HTMLMediaElement::GetDocument() const
+{
+  return OwnerDoc();
+}
+
 bool HasDebuggerPrivilege(JSContext* aCx, JSObject* aObj)
 {
   return nsContentUtils::CallerHasPermission(aCx,