[Android] Support hardware accelerated vp8 encoding for WebRTC when available

This patch is back ported from chromium trunk, please see:
https://codereview.chromium.org/74563002/
https://codereview.chromium.org/106333002/
Please remove this patch if chromium is rebased to version 33.

BUG=https://crosswalk-project.org/jira/browse/XWALK-611