MediaStreamTrack: Add |mute| attribute and connection.

This patch adds |muted| read only property [1] to MediaStreamTrack 
[2] as per W3C doco.

[1] http://dev.w3.org/2011/webrtc/editor/getusermedia.html#widl-MediaStreamTrack-muted
[2] http://dev.w3.org/2011/webrtc/editor/getusermedia.html#media-stream-track-interface-definition

BUG=389159

Review URL: https://codereview.chromium.org/397603002

git-svn-id: svn://svn.chromium.org/blink/trunk@178337 bbb929c8-8fbe-4397-9dbb-9b2b20218538