webrtc.js: handle invalid ICE candidates gracefully

On invalid ICE candidates invoke the failureCallback passed to addIceCandidate.
Also call completeQueuedOperation() so that the internal operation queue can
proceed and doesn't get stuck.
webrtc.js: Work around Chrome WebRTC bug #1142

https://code.google.com/p/webrtc/issues/detail?id=1142

Fixes interop with Chrome Android v37.0.2062.117, for example.

The RTCIceCandidate.candidate attribute as passed by Chrome for Android is of
the format `a=candidate-attribute', meaning that the actual candidate is
prefixed by a=.