Bug 1244768 part 6 - modify the internal pause steps; r=jwwang

4.8.12.8
https://html.spec.whatwg.org/multipage/embedded-content.html#internal-pause-steps

MozReview-Commit-ID: Dt0VVbXLrhw

dom/html/HTMLMediaElement.cpp

@@ -2747,6 +2747,7 @@ HTMLMediaElement::Pause(ErrorResult& aRv)
   if (!oldPaused) {
     FireTimeUpdate(false);
     DispatchAsyncEvent(NS_LITERAL_STRING("pause"));
+    AsyncRejectPendingPlayPromises(NS_ERROR_DOM_MEDIA_ABORT_ERR);
   }
 }