Implement sequence<MediaStreamTrack> getTracks () This is to conform to the new spec- http://dev.w3.org/2011/webrtc/editor/getusermedia.html#mediastream

BUG=394212

Review URL: https://codereview.chromium.org/397903003

git-svn-id: svn://svn.chromium.org/blink/trunk@178338 bbb929c8-8fbe-4397-9dbb-9b2b20218538