[webapi][XWALK-2592] Fix bug for webrtc

- Fix bug for webrtc

Impacted tests(designed): new 0, update 1, delete 0
Unit test platform: [Tizen IVI]
Unit test result summary: pass 1, fail 0, block 0
Merge pull request #955 from yunxliu/yunxliu/webapi/webrtc_xwalk2592

[webapi][XWALK-2592] Fix bug for webrtc