Revert 6766 "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."

BUG=N/A
TBR=ajm@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/18899004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6769 4adac7df-926f-26a2-2b94-8c16560cd09d
Revert 6747 "Refactor StatsCollector and associated types."
Breakes FYI bots.

BUG=N/A
TBR=ajm@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/18889004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6770 4adac7df-926f-26a2-2b94-8c16560cd09d