Volume buttons in AppRTCDemo should affect output audio volume (part II).

See https://webrtc-codereview.appspot.com/32399004/ for part I.

BUG=3279
TEST=AppRTC demo
R=perkj@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/27059004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7654 4adac7df-926f-26a2-2b94-8c16560cd09d