[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 #1485 from yunxliu/webapi/webrtc_2593

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