Fix the issue getUserMedia can't work if rebasing to Chromium 31

Crosswalk debug mode crashed when it accessed getUserMedia API, and
release mode also can't work.

The root cause was listed as followed:
1. Chromium upstream improved related code, now EnsureMonitorCaptureDevices
   is just a wrapper, it runs in UI thread, and the real major code is
   still running in IO thread.
   Please see: https://codereview.chromium.org/17508005/
2. It is an error from rebasing, GetMediaObserver callback was changed
   to be NULL.