[usecase-webapi] Improve WebRTC usecase

- Adjust the case with screen view
- Failure analysis: https://crosswalk-project.org/jira/browse/XWALK-1535

Impacted tests(approved): new 0, update 1, delete 0
Unit test platform: [Tizen IVI]
Unit test result summary: pass 0, fail 1, block 0
Merge pull request #593 from yunxliu/yunxliu/misc/webrtc_update

[usecase-webapi] Improve WebRTC usecase