[webapi][xwalk-2561] Update codes based on Chromium implement

- mp4a.40.8/mp4v.20.8 is also not supported in chromium 38, need use mp4a.40.2/mp4a.40 instead.
- Verified on Crosswalk 9.38.204.0

Impacted tests(approved): new 0, update 2, delete 0
Unit test platform: [Tizen IVI][Android]
Unit test result summary: pass 2, fail 0, block 0
Merge pull request #933 from haoxli/haoxli/webapi/xwalk-2561-video

[webapi][xwalk-2561] Update codes based on Chromium implement