Add locks to Start(), Stop() methods in ProcessThread.
This is necessary unfortunately since there are a few places where DeRegisterModule does not reliably occur on the same thread.

BUG=4473
R=pbos@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/42979004

Cr-Commit-Position: refs/heads/master@{#8891}
1 file changed