Bug 1511235 - part2 : add test. r=jya,baku

Add new webidl method for testing only and a test.

Differential Revision: https://phabricator.services.mozilla.com/D13805

dom/html/HTMLMediaElement.cpp

@@ -1631,6 +1631,10 @@ void HTMLMediaElement::SetVisible(bool aVisible) {
   }
 }
 
+bool HTMLMediaElement::IsVideoDecodingSuspended() const {
+  return mDecoder && mDecoder->IsVideoDecodingSuspended();
+}
+
 already_AddRefed<layers::Image> HTMLMediaElement::GetCurrentImage() {
   MarkAsTainted();