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..150c441a7580649ffa6e95456beb955aa61bb038 100644
--- a/base/message_loop/message_loop_proxy_impl.h
+++ b/base/message_loop/message_loop_proxy_impl.h
@@ -21,7 +21,14 @@ class IncomingTaskQueue;
 // MessageLoop.
 class BASE_EXPORT MessageLoopProxyImpl : public MessageLoopProxy {
  public:
-  explicit MessageLoopProxyImpl(
+  static MessageLoopProxyImpl* Create(
+      scoped_refptr<IncomingTaskQueue> incoming_queue);
+
+  // Creates a proxy for lazy initialization. This type of MessageLoopProxy
+  // is intended to be created for the message loop that is to be initialized
+  // lazily (i.e. created by CreateForLazyInit()).  See the comment at
+  // MessageLoop::CreateForLazyInit() for more details.
+  static MessageLoopProxyImpl* CreateForLazyInit(
       scoped_refptr<IncomingTaskQueue> incoming_queue);
 
   // MessageLoopProxy implementation
@@ -33,8 +40,15 @@ class BASE_EXPORT MessageLoopProxyImpl : public MessageLoopProxy {
                                   base::TimeDelta delay) override;
   bool RunsTasksOnCurrentThread() const override;
 
+  // Initialize this message loop proxy on the current thread.
+  // It is not valid to call this unless this proxy is created by
+  // CreateForLazyInit().
+  void LazyInit();
+
  private:
   friend class RefCountedThreadSafe<MessageLoopProxyImpl>;
+  MessageLoopProxyImpl(scoped_refptr<IncomingTaskQueue> incoming_queue,
+                       bool lazy_init);
   ~MessageLoopProxyImpl() override;
 
   // THe incoming queue receiving all posted tasks.