Bug 1299718 part 3 - call MarkAsContentSource() at where using video element as a source; r=gerald,kamidphish,mtseng

MozReview-Commit-ID: Kg6UpcsCi0P

dom/html/HTMLMediaElement.cpp

@@ -2546,6 +2546,8 @@ HTMLMediaElement::CaptureStreamInternal(bool aFinishWhenEnded,
 {
   MOZ_RELEASE_ASSERT(aGraph);
 
+    MarkAsContentSource(CallerAPI::CAPTURE_STREAM);
+
   nsPIDOMWindowInner* window = OwnerDoc()->GetInnerWindow();
   if (!window) {
     return nullptr;