mus: Use TooltipManagerAura in NativeWidgetMus.

ui/views/mus/screen_mus_delegate.h

 // Copyright 2016 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 UI_VIEWS_MUS_SCREEN_MUS_DELEGATE_H_
 #define UI_VIEWS_MUS_SCREEN_MUS_DELEGATE_H_
 
 #include "ui/views/mus/mus_export.h"
 
 namespace gfx {
 class Point;
 }
 
+namespace ui {
+class Window;
+}
+
 namespace views {
 
 // Screen implementation backed by ui::mojom::DisplayManager.
 class VIEWS_MUS_EXPORT ScreenMusDelegate {
  public:
   virtual void OnWindowManagerFrameValuesChanged() = 0;
 
   virtual gfx::Point GetCursorScreenPoint() = 0;
 
-protected:
+  virtual ui::Window* GetWindowAtScreenPoint(const gfx::Point& point) = 0;
+
+ protected:
   virtual ~ScreenMusDelegate() {}
 };
 
 }  // namespace views
 
 #endif  // UI_VIEWS_MUS_SCREEN_MUS_DELEGATE_H_