Allow WebRtcMediaEngine to be created from any thread.

webrtc/media/engine/webrtcmediaengine.h

Index: webrtc/media/engine/webrtcmediaengine.h
diff --git a/webrtc/media/engine/webrtcmediaengine.h b/webrtc/media/engine/webrtcmediaengine.h
index 1fc988d1f45299b430526b05431b9ecdf48569f4..db955d0a72aa890b7dcf4f7947e9c8e6c1d93f9e 100644
--- a/webrtc/media/engine/webrtcmediaengine.h
+++ b/webrtc/media/engine/webrtcmediaengine.h
@@ -32,6 +32,10 @@ namespace cricket {
 
 class WebRtcMediaEngineFactory {
  public:
+  // These Create methods may be called on any thread, though the engine is
+  // only expected to be used on one thread, internally called the "worker
+  // thread". This is the thread Init must be called on.
+
   // TODO(ossu): Backwards-compatible interface. Will be deprecated once the
   // audio decoder factory is fully plumbed and used throughout WebRTC.
   // See: crbug.com/webrtc/6000