[Crosswalk-9][webapi][XWALK-2593] Fix bug for webrtc

- Fix bug for webrtc
- `addIceCandidate("", function () {}, function () {})` should throw `TypeError`

Impacted tests(approved): new 0, update 1, delete 0
Unit test platform: [Tizen IVI]
Unit test result summary: pass 1, fail 0, block 0
Merge pull request #1084 from yunxliu/Crosswalk-9

[Crosswalk-9][webapi][XWALK-2593] Fix bug for webrtc