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

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 0fe629f9d140c0244128e49598308a9a48d15366..fa611c290853451f9779a9cb35699c7c94aff763 100644
--- a/base/message_loop/message_loop_proxy_impl.h
+++ b/base/message_loop/message_loop_proxy_impl.h
@@ -9,6 +9,7 @@
 #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 {
@@ -24,6 +25,9 @@ class BASE_EXPORT MessageLoopProxyImpl : public MessageLoopProxy {
   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,
@@ -40,8 +44,10 @@ class BASE_EXPORT MessageLoopProxyImpl : public MessageLoopProxy {
   // 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);
 };