Lock resources in event_posix.cc.

Fixes errors reported by Helgrind from event_posix.cc when running video_engine_tests.

BUG=
TEST=helgrind,trybots
R=henrike@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/2060004

git-svn-id: http://webrtc.googlecode.com/svn/trunk/webrtc@4572 4adac7df-926f-26a2-2b94-8c16560cd09d
1 file changed