mute audio in paused state

[fpn]

gst-cef support the PAUSE state - in this case the browser goes into background mode and stops rendering new content. This significantly reduces resource usage (mainly CPU). Audio is still produced to the system though.

In most use cases people don't expect audio in paused state ;-).

https://bitbucket.org/chromiumembedded/cef/pull-requests/173/mute-audio-in-the-browser-issue-1806/diff