mus: Use the new drag API to implement tab dragging in chrome.

ash/mus/frame/move_event_handler.h

 // Copyright 2015 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 ASH_MUS_FRAME_MOVE_EVENT_HANDLER_H_
 #define ASH_MUS_FRAME_MOVE_EVENT_HANDLER_H_
 
 #include <memory>
 
 #include "ash/common/wm/wm_toplevel_window_event_handler.h"
@@ skipping 21 matching lines @@
 
 // EventHandler attached to windows that may be dragged and/or resized. This
 // forwards to WmToplevelWindowEventHandler to handle the actual drag/resize.
 class MoveEventHandler : public ui::EventHandler, public aura::WindowObserver {
  public:
   MoveEventHandler(::mus::Window* mus_window,
                    ::mus::WindowManagerClient* window_manager_client,
                    aura::Window* aura_window);
   ~MoveEventHandler() override;
 
+  // Retrieves the MoveEventHandler for an existing WmWindow.
+  static MoveEventHandler* GetForWindow(WmWindow* wm_window);
+
+  // Attempts to start a drag if one is already not in progress. This passes

[sky, 2016/06/24 19:42:02]

nit: 'already not' -> 'not alreay'

+  // the call to the underlying WmToplevelWindowEventHandler.

[sky, 2016/06/24 19:42:02]

document what end_closure is.

+  void AttemptToStartDrag(
+      const gfx::Point& point_in_parent,
+      int window_component,
+      const base::Callback<void(bool success)>& end_closure);
+
+  // Reverts a manually started drag started with AttemptToStartDrag().
+  void RevertDrag();
+
  private:
   // Removes observer and EventHandler installed on |root_window_|.
   void Detach();
 
   // Overridden from ui::EventHandler:
   void OnMouseEvent(ui::MouseEvent* event) override;
   void OnGestureEvent(ui::GestureEvent* event) override;
   void OnCancelMode(ui::CancelModeEvent* event) override;
 
   // Overridden from aura::WindowObserver:
   void OnWindowDestroying(aura::Window* window) override;
 
   WmWindowMus* wm_window_;
   ::mus::WindowManagerClient* window_manager_client_;
   // The root window of the aura::Window supplied to the constructor.
   // MoveEventHandler is added as a pre-target handler (and observer) of this.
   aura::Window* root_window_;
   wm::WmToplevelWindowEventHandler toplevel_window_event_handler_;
 
   DISALLOW_COPY_AND_ASSIGN(MoveEventHandler);
 };
 
 }  // namespace mus
 }  // namespace ash
 
 #endif  // ASH_MUS_FRAME_MOVE_EVENT_HANDLER_H_