Made MessagePump a non-thread safe class.

base/message_loop/message_loop_proxy_impl.h

Index: base/message_loop/message_loop_proxy_impl.h
diff --git a/base/message_loop/message_loop_proxy_impl.h b/base/message_loop/message_loop_proxy_impl.h
index 2269023ac2d19c0f9074fcadf181a3a9da1f3f51..575365101bc6e00351d49e6edd61cb5740e62941 100644
--- a/base/message_loop/message_loop_proxy_impl.h
+++ b/base/message_loop/message_loop_proxy_impl.h
@@ -37,6 +37,7 @@ class BASE_EXPORT MessageLoopProxyImpl : public MessageLoopProxy {
  private:
   // Allow the MessageLoop to create a MessageLoopProxyImpl.
   friend class MessageLoop;
+  friend class MessageLoopLockTest;
   friend class DeleteHelper<MessageLoopProxyImpl>;
 
   MessageLoopProxyImpl();
@@ -44,11 +45,23 @@ class BASE_EXPORT MessageLoopProxyImpl : public MessageLoopProxy {
   // Called directly by MessageLoop::~MessageLoop.
   virtual void WillDestroyCurrentMessageLoop();
 
+  // Takes |message_loop_lock_| and posts a task to |target_message_loop_|.
+  // Posting of all tasks is routed though one these functions ensuring that
+  // |message_loop_lock_| is always while modifying the incoming queue.
+  //
+  // Returns true if the task was successfully added to the queue, otherwise
+  // returns false. In all cases, the ownership of |task| is transferred to the
+  // called method.
+  bool AddToIncomingQueue(const tracked_objects::Location& from_here,
+                          const Closure& task,
+                          TimeDelta delay,
+                          bool nestable);
 
-  bool PostTaskHelper(const tracked_objects::Location& from_here,
-                      const base::Closure& task,
-                      base::TimeDelta delay,
-                      bool nestable);
+  // Same as AddToIncomingQueue() except that it will avoid blocking if the lock
+  // is already held, and will in that case (when the lock is contended) fail to
+  // add the task, and will return false.
+  bool TryAddToIncomingQueue(const tracked_objects::Location& from_here,
+                             const Closure& task);
 
   // The lock that protects access to target_message_loop_.
   mutable base::Lock message_loop_lock_;