Bug 1573524 - In MediaRecorder::MediaStreamReady, get the graph rate from a track that's guaranteed live. r=padenot

Differential Revision: https://phabricator.services.mozilla.com/D42525

--HG--
extra : moz-landing-system : lando

dom/media/MediaRecorder.cpp

@@ -714,7 +714,7 @@ class MediaRecorder::Session : public PrincipalChangeObserver<MediaStreamTrack>,
 
     LOG(LogLevel::Debug,
         ("Session.MediaTracksReady track type = (%d)", trackTypes));
-    InitEncoder(trackTypes, tracks[0]->Graph()->GraphRate());
+    InitEncoder(trackTypes, mMediaStreamTracks[0]->Graph()->GraphRate());
   }
 
   void ConnectMediaStreamTrack(MediaStreamTrack& aTrack) {