Protect MessageQueue stop field with a critical section to avoid data races.

webrtc/base/messagequeue.h

Index: webrtc/base/messagequeue.h
diff --git a/webrtc/base/messagequeue.h b/webrtc/base/messagequeue.h
index a1b20da0e5194ac849b21b5a579418deff04ec1a..628ce4433b824396c952400836d39bd8df023a6c 100644
--- a/webrtc/base/messagequeue.h
+++ b/webrtc/base/messagequeue.h
@@ -288,7 +288,6 @@ class MessageQueue {
 
   void WakeUpSocketServer();
 
-  bool fStop_;
   bool fPeekKeep_;
   Message msgPeek_;
   MessageList msgq_ GUARDED_BY(crit_);
@@ -299,6 +298,8 @@ class MessageQueue {
   bool fDestroyed_;
 
  private:
+  volatile int stop_;
+
   // The SocketServer might not be owned by MessageQueue.
   SocketServer* ss_ GUARDED_BY(ss_lock_);
   // Used if SocketServer ownership lies with |this|.