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 3a5226cd0a0129bb6c149b170922eef6123b3614..8f477f933dd8c480cb8ec946377173cddc7df497 100644
--- a/webrtc/base/messagequeue.h
+++ b/webrtc/base/messagequeue.h
@@ -280,7 +280,6 @@ class MessageQueue {
 
   void WakeUpSocketServer();
 
-  bool fStop_;
   bool fPeekKeep_;
   Message msgPeek_;
   MessageList msgq_ GUARDED_BY(crit_);
@@ -291,6 +290,9 @@ class MessageQueue {
   bool fDestroyed_;
 
  private:
+  bool stop_ GUARDED_BY(stop_crit_);
+  CriticalSection stop_crit_ ACQUIRED_AFTER(crit_);
+
   // The SocketServer might not be owned by MessageQueue.
   SocketServer* ss_ GUARDED_BY(ss_lock_);
   // Used if SocketServer ownership lies with |this|.