Adding an audio-level WebRTC test to verify muting and playback.

This test uses the stats API to measure the audioOutputLevel. Since the
content browsertests always uses fake devices we can adapt the test to
look for that particular signal.

This test is primarily intended to run on Android but will work on 
desktop as well. Tested by running manually on a Nexus 7 device 
at my workstation.

BUG=325164

Review URL: https://codereview.chromium.org/101773002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238953 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238953