Bug 1329075 - Avoid an infinite event loop spin. r=jesup

Because we add tracks to the output streams async, it's possible to switch the
mSrcStream of a media element and *then* get a notification of an added track,
when this track originated from the old mSrcStream.
If the new mSrcStream is an output stream of the media element, this would
again add a new track async, which on the next event loop spin would show up
on mSrcStream, and the loop continues.

MozReview-Commit-ID: HmKgXLYmubh

dom/html/HTMLMediaElement.cpp

@@ -3150,6 +3150,12 @@ HTMLMediaElement::AddCaptureMediaTrackToOutputStream(MediaTrack* aTrack,
   }
   aOutputStream.mCapturingMediaStream = true;
 
+  if (aOutputStream.mStream == mSrcStream) {
+    // Cycle detected. This can happen since tracks are added async.
+    // We avoid forwarding it to the output here or we'd get into an infloop.
+    return;
+  }
+
   MediaStream* outputSource = aOutputStream.mStream->GetInputStream();
   if (!outputSource) {
     NS_ERROR("No output source stream");