rtc::CriticalSection is now lockable from const methods and no longer need to remain mutable. BUG= R=tommi@webrtc.org Review URL: https://codereview.webrtc.org/1613643004 Cr-Commit-Position: refs/heads/master@{#11367}
BUG=webrtc:4690 Review URL: https://codereview.webrtc.org/1403363003 Cr-Commit-Position: refs/heads/master@{#10548}