ProcessThreadImpl - hold the lock while checking thread_ and calling ProcessThreadAttached(). This is needed since DeRegisterModule is currently being called on arbitrary threads.
ProcessThreadImpl - hold the lock while checking thread_ and calling ProcessThreadAttached().
This is needed since DeRegisterModule is currently being called on arbitrary threads.

BUG=4508
R=perkj@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/48829004

Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#8924}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 7f375f0ef8101c7f24f90067759bfa84b35f1c87

sippet/webrtc,sippet/webrtc,sippet/webrtc,sippet/webrtc,sippet/webrtc,sippet/webrtc