[webapi][XWALK-2593] Fix bug for webrtc

- Fix bug for webrtc
- Failure analysis: https://crosswalk-project.org/jira/browse/XWALK-2593

Impacted tests(approved): new 0, update 1, delete 0
Unit test platform: [Tizen IVI]
Unit test result summary: pass 0, fail 1, block 0
Merge pull request #953 from yunxliu/yunxliu/webapi/webrtc_xwalk2593

[webapi][XWALK-2593] Fix bug for webrtc