[usecase-webapi][XWALK-1473] Fix bug for WebRTC

- The onDataChannel can not be fired on the first channel inChrom, so avoid to use it

Impacted tests(approved): new 0, update 1, delete 0
Unit test platform: [Android]
Unit test result summary: pass 1, fail 0, block 0
Merge pull request #711 from yunxliu/yunxliu/misc/webrtc_fix

[usecase-webapi][XWALK-1473] Fix bug for WebRTC