Reland (3rd try): Lazily initialize MessageLoop for faster thread startup

base/message_loop/message_loop_proxy_impl.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef BASE_MESSAGE_LOOP_MESSAGE_LOOP_PROXY_IMPL_H_
 #define BASE_MESSAGE_LOOP_MESSAGE_LOOP_PROXY_IMPL_H_
 
 #include "base/base_export.h"
 #include "base/memory/ref_counted.h"
 #include "base/message_loop/message_loop_proxy.h"
 #include "base/pending_task.h"
+#include "base/synchronization/lock.h"
 #include "base/threading/platform_thread.h"
 
 namespace base {
 namespace internal {
 
 class IncomingTaskQueue;
 
 // A stock implementation of MessageLoopProxy that is created and managed by a
 // MessageLoop. For now a MessageLoopProxyImpl can only be created as part of a
 // MessageLoop.
 class BASE_EXPORT MessageLoopProxyImpl : public MessageLoopProxy {
  public:
   explicit MessageLoopProxyImpl(
       scoped_refptr<IncomingTaskQueue> incoming_queue);
 
+  // Initialize this message loop proxy on the current thread.
+  void BindToCurrentThread();
+
   // MessageLoopProxy implementation
   bool PostDelayedTask(const tracked_objects::Location& from_here,
                        const base::Closure& task,
                        base::TimeDelta delay) override;
   bool PostNonNestableDelayedTask(const tracked_objects::Location& from_here,
                                   const base::Closure& task,
                                   base::TimeDelta delay) override;
   bool RunsTasksOnCurrentThread() const override;
 
  private:
   friend class RefCountedThreadSafe<MessageLoopProxyImpl>;
   ~MessageLoopProxyImpl() override;
 
   // THe incoming queue receiving all posted tasks.
   scoped_refptr<IncomingTaskQueue> incoming_queue_;
 
-  // ID of the thread |this| was created on.
+  // ID of the thread |this| was created on.  Could be accessed on multiple
+  // threads, protected by |valid_thread_id_lock_|.
   PlatformThreadId valid_thread_id_;
+  mutable Lock valid_thread_id_lock_;
 
   DISALLOW_COPY_AND_ASSIGN(MessageLoopProxyImpl);
 };
 
 }  // namespace internal
 }  // namespace base
 
 #endif  // BASE_MESSAGE_LOOP_MESSAGE_LOOP_PROXY_IMPL_H_