[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
I guess this is misuse of `assert_throws`, see https://github.com/w3c/testharness.js/blob/master/docs/api.md#assert_throwscode-func-description

Because I don't think the function defined by lines 69-78 is able to throw a "SyntaxError" exception. You can double check this by `try .. catch`.