Use atomic operations for setting/reading the trace filter.
The filter is currently being set and read by a number of threads and tripping up tsan.

R=mflodman@webrtc.org
BUG=

Review URL: https://webrtc-codereview.appspot.com/47609004

Cr-Commit-Position: refs/heads/master@{#8753}
git-svn-id: http://webrtc.googlecode.com/svn/trunk@8753 4adac7df-926f-26a2-2b94-8c16560cd09d
3 files changed