[webapi] Add back 2 tests for WebRTC and remove duplicated ones

Impacted Suites: webapi-webrtc-w3c-tests
Impacted TCs num: New 2, Update 0, Delete 2
Unit test platform: IVI
IVI test result summary: Pass 0, Fail 2, Blocked 0

Add Reason:
Add 2 TCs for missing test point.

Fail Reason:
The attribute setIdentityProvider and getIdentityAssertion are not support.

Remove TC check point repeat:
RTCPeerConnection_iceGatheringState.html file is covered by https://github.com/crosswalk-project/crosswalk-test-suite/tree/master/webapi/webapi-webrtc-w3c-tests/webrtc/RTCPeerConnection_basic.html.
RTCPeerConnection_negotiationneeded_event.html file no reference and is duplicated by https://github.com/crosswalk-project/crosswalk-test-suite/tree/master/webapi/webapi-webrtc-w3c-tests/webrtc/blink/RTCPeerConnection-onnegotiationneeded.html.