Bug 1329075 - Fix assertion failure in debug build. r=jesup

With the attached test case I was failing an assert in a debug build because
NotifyMediaTrackEnabled exited early due to `mReadyState == HAVE_NOTHING` but
NotifyMediaTrackDisabled didn't. The latter would fail an assert.

MozReview-Commit-ID: 7Frpj62s0Bc

--HG--
extra : rebase_source : a1dea4f2db564fe28e6eb7f7fcd9c382a46409ac

dom/html/HTMLMediaElement.cpp

@@ -2023,6 +2023,12 @@ void HTMLMediaElement::NotifyMediaTrackDisabled(MediaTrack* aTrack)
     }
   }
 
+  if (mReadyState == HAVE_NOTHING) {
+    // No MediaStreamTracks are captured until we have metadata, and code
+    // below doesn't do anything for captured decoders.
+    return;
+  }
+
   for (OutputMediaStream& ms : mOutputStreams) {
     if (ms.mCapturingDecoder) {
       MOZ_ASSERT(!ms.mCapturingMediaStream);