Reusing base::IdType<...> to implement SurfaceId.

components/mus/ws/window_finder.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 COMPONENTS_MUS_WS_WINDOW_FINDER_H_
 #define COMPONENTS_MUS_WS_WINDOW_FINDER_H_
 
-namespace cc {
-struct SurfaceId;
-}
+#include "cc/surfaces/surface_id.h"
 
 namespace gfx {
 class Point;
 class Transform;
 }
 
 namespace mus {
 namespace ws {
 
 class ServerWindow;
 
 // Find the deepest visible child of |root| that should receive an event at
 // |location|. |location| is initially in the coordinate space of
 // |root_window|, on return it is converted to the coordinates of the return
 // value.
 ServerWindow* FindDeepestVisibleWindowForEvents(
     ServerWindow* root_window,
     cc::SurfaceId display_surface_id,
     gfx::Point* location);
 
 // Retrieve the transform to the provided |window|'s coordinate space from the
 // root.
 gfx::Transform GetTransformToWindow(cc::SurfaceId display_surface_id,
                                     ServerWindow* window);
 
 }  // namespace ws
 }  // namespace mus
 
 #endif  // COMPONENTS_MUS_WS_WINDOW_FINDER_H_