Thread annotation of talk_base::CriticalSection.

Also enabling -Wthread-safety in talk/build/common.gypi for clang on
Linux. Thread annotations are compile-time checks that for instance
certain locks are held before accessing a value.

BUG=
TEST=Local GUARDED_BY() annotations.
R=andresp@webrtc.org, fischman@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/8189004

git-svn-id: http://webrtc.googlecode.com/svn/trunk/talk@5516 4adac7df-926f-26a2-2b94-8c16560cd09d
5 files changed