[webapi] Fix XWALK-1447 to define a variable

Impacted test suite: webapi-webrtc-w3c-tests
Impacted TCs num: New 0, Update 1, Delete 0
Unit test platform: IVI
IVI test result summary: Fail

Fail Reason:
Fix XWALK-1447 bug to define a variable, but this case is still failed due to the ondatachannel event cannot be fired.