Make MediaOptimization thread-safe.

HW encoder posts the encode callback to libjingle worker
thread. It accesses MediaOptimization and is not protected
by the critial section of VideoSender. Make MediaOptimization
thread-safe to fix it.

BUG=chromium:367691
TEST=Run apprtc loopback with SW or HW encoders.
     Run module_unittests.

R=stefan@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/12849004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6562 4adac7df-926f-26a2-2b94-8c16560cd09d