Temporarily add a default ctor to StatsReport and make |id| non const.
As soon as I've updated the chrome side, I'll revert this cl.

TBR=henrike

Review URL: https://webrtc-codereview.appspot.com/16149004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6766 4adac7df-926f-26a2-2b94-8c16560cd09d