Update from https://crrev.com/328418

base/message_loop/message_loop.h

Index: base/message_loop/message_loop.h
diff --git a/base/message_loop/message_loop.h b/base/message_loop/message_loop.h
index fd7596a79204ed89291f32b2f9135108a73b2882..f2f89d0574dc05f70102cc592fa0250860e13031 100644
--- a/base/message_loop/message_loop.h
+++ b/base/message_loop/message_loop.h
@@ -114,7 +114,8 @@ class BASE_EXPORT MessageLoop : public MessagePump::Delegate {
   explicit MessageLoop(Type type = TYPE_DEFAULT);
   // Creates a TYPE_CUSTOM MessageLoop with the supplied MessagePump, which must
   // be non-NULL.
-  explicit MessageLoop(scoped_ptr<base::MessagePump> pump);
+  explicit MessageLoop(scoped_ptr<MessagePump> pump);
+
   ~MessageLoop() override;
 
   // Returns the MessageLoop object for the current thread, or null if none.
@@ -394,10 +395,6 @@ class BASE_EXPORT MessageLoop : public MessagePump::Delegate {
   // Returns true if the message loop is "idle". Provided for testing.
   bool IsIdleForTesting();
 
-  // Wakes up the message pump. Can be called on any thread. The caller is
-  // responsible for synchronizing ScheduleWork() calls.
-  void ScheduleWork();
-
   // Returns the TaskAnnotator which is used to add debug information to posted
   // tasks.
   debug::TaskAnnotator* task_annotator() { return &task_annotator_; }
@@ -411,9 +408,33 @@ class BASE_EXPORT MessageLoop : public MessagePump::Delegate {
 
  private:
   friend class RunLoop;
+  friend class internal::IncomingTaskQueue;
+  friend class ScheduleWorkTest;
+  friend class Thread;
+
+  using MessagePumpFactoryCallback = Callback<scoped_ptr<MessagePump>()>;
 
-  // Configures various members for the two constructors.
-  void Init();
+  // Creates a MessageLoop without binding to a thread.
+  // If |type| is TYPE_CUSTOM non-null |pump_factory| must be also given
+  // to create a message pump for this message loop.  Otherwise a default
+  // message pump for the |type| is created.
+  //
+  // It is valid to call this to create a new message loop on one thread,
+  // and then pass it to the thread where the message loop actually runs.
+  // The message loop's BindToCurrentThread() method must be called on the
+  // thread the message loop runs on, before calling Run().
+  // Before BindToCurrentThread() is called only Post*Task() functions can
+  // be called on the message loop.
+  scoped_ptr<MessageLoop> CreateUnbound(
+      Type type,
+      MessagePumpFactoryCallback pump_factory);
+
+  // Common private constructor. Other constructors delegate the initialization
+  // to this constructor.
+  MessageLoop(Type type, MessagePumpFactoryCallback pump_factory);
+
+  // Configure various members and bind this message loop to the current thread.
+  void BindToCurrentThread();
 
   // Invokes the actual run loop using the message pump.
   void RunHandler();
@@ -437,6 +458,10 @@ class BASE_EXPORT MessageLoop : public MessagePump::Delegate {
   // empty.
   void ReloadWorkQueue();
 
+  // Wakes up the message pump. Can be called on any thread. The caller is
+  // responsible for synchronizing ScheduleWork() calls.
+  void ScheduleWork();
+
   // Start recording histogram info about events and action IF it was enabled
   // and IF the statistics recorder can accept a registration of our histogram.
   void StartHistogrammer();
@@ -490,6 +515,10 @@ class BASE_EXPORT MessageLoop : public MessagePump::Delegate {
   bool os_modal_loop_;
 #endif
 
+  // pump_factory_.Run() is called to create a message pump for this loop
+  // if type_ is TYPE_CUSTOM and pump_ is null.
+  MessagePumpFactoryCallback pump_factory_;
+
   std::string thread_name_;
   // A profiling histogram showing the counts of various messages and events.
   HistogramBase* message_histogram_;