[webrtc] Add permission for WebRTC audio channel
[webrtc] Add permission for WebRTC audio channel

Permission MODIFY_AUDIO_SETTINGS is also needed for audio channel in WebRTC

BUG=https://crosswalk-project.org/jira/browse/XWALK-6069

haoxli/crosswalk-samples,crosswalk-project/crosswalk-samples,crosswalk-project/crosswalk-samples,crosswalk-project/crosswalk-samples,haoxli/crosswalk-samples,crosswalk-project/crosswalk-samples,haoxli/crosswalk-samples,crosswalk-project/crosswalk-samples,haoxli/crosswalk-samples