Do not require synchronization access on the thread if called from rtc::Thread::WrapCurrent.
The synchronization access is unnecessary for rtc::Thread::WrapCurrent (called from JingleThreadWrapper) since JingleThreadWrapper never calls rtc::Thread::Stop or rtc::Thread::Join. Failing to get the access caused crashes in Chrome since rtc::Thread::Current will be NULL when rtc::Thread::WrapCurrent fails.

rtc::ThreadManager::WrapCurrentThread still requires the synchronization access, since I am not sure if the callers (e.g. the plugin) depends on it.

BUG=crbug/413853
R=juberti@webrtc.org, tommi@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/30429004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7224 4adac7df-926f-26a2-2b94-8c16560cd09d