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,
   please see: https://codereview.chromium.org/17508005/
2. It is an error from rebasing, GetMediaObserver callback was changed to be NULL.