Bug 1208378 - Make HTMLMediaElement register its Sink. r=jib

This is a drive-by fix in that it is not directly related to what the bug is
solving. However, making HTMLMediaElement register as a sink is important,
and pairing it with the WeakPtr<Sink> patch reduces risk greatly.

MozReview-Commit-ID: 7pMDw3MG0ZB

dom/html/HTMLMediaElement.cpp

@@ -3158,6 +3158,8 @@ public:
     MOZ_ASSERT(mCapturedTrackSource);
     MOZ_ASSERT(mOwningStream);
     MOZ_ASSERT(IsTrackIDExplicit(mDestinationTrackID));
+
+    mCapturedTrackSource->RegisterSink(this);
   }
 
   void Destroy() override