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

webrtc/base/thread.cc

Index: webrtc/base/thread.cc
diff --git a/webrtc/base/thread.cc b/webrtc/base/thread.cc
index af5e68bedf81234704fa3d8e611d196f13865b90..5278c9321eae369167fb5717d5be84c6caa16021 100644
--- a/webrtc/base/thread.cc
+++ b/webrtc/base/thread.cc
@@ -218,7 +218,7 @@
   ASSERT(!running());
   if (running()) return false;
 
-  Restart();  // reset IsQuitting() if the thread is being restarted
+  Restart();  // reset fStop_ if the thread is being restarted
 
   // Make sure that ThreadManager is created on the main thread before
   // we start a new thread.
@@ -346,7 +346,7 @@
                   MessageHandler* phandler,
                   uint32_t id,
                   MessageData* pdata) {
-  if (IsQuitting())
+  if (fStop_)
     return;
 
   // Sent messages are sent to the MessageHandler directly, in the context