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

- Fix bug for webrtc
- Failure analysis: Feature issue, the candidate parameter is malformed, should throw a SyntaxError exception by spec, but throw a TypeMismatchError by test

Impacted tests(designed): new 0, update 1, delete 0
Unit test platform: [Tizen IVI]
Unit test result summary: pass 0, fail 1, block 0