Mojo C++ bindings: support mapping T and T? differently.

services/ui/ws/window_tree.h

 // Copyright 2014 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 SERVICES_UI_WS_WINDOW_TREE_H_
 #define SERVICES_UI_WS_WINDOW_TREE_H_
 
 #include <stdint.h>
 
 #include <map>
@@ skipping 392 matching lines @@
                                mojo::TextInputStatePtr state) override;
   void SetImeVisibility(Id transport_window_id,
                         bool visible,
                         mojo::TextInputStatePtr state) override;
   void OnWindowInputEventAck(uint32_t event_id,
                              mojom::EventResult result) override;
   void SetClientArea(
       Id transport_window_id,
       const gfx::Insets& insets,
       mojo::Array<gfx::Rect> transport_additional_client_areas) override;
-  void SetHitTestMask(Id transport_window_id, const gfx::Rect& mask) override;
+  void SetHitTestMask(Id transport_window_id,
+                      const base::Optional<gfx::Rect>& mask) override;
   void GetWindowManagerClient(
       mojo::AssociatedInterfaceRequest<mojom::WindowManagerClient> internal)
       override;
   void GetCursorLocationMemory(const GetCursorLocationMemoryCallback& callback)
       override;
   void PerformWindowMove(uint32_t change_id,
                          Id window_id,
                          ui::mojom::MoveLoopSource source,
                          const gfx::Point& cursor) override;
   void CancelWindowMove(Id window_id) override;
@@ skipping 81 matching lines @@
       waiting_for_top_level_window_info_;
   bool embedder_intercepts_events_ = false;
 
   DISALLOW_COPY_AND_ASSIGN(WindowTree);
 };
 
 }  // namespace ws
 }  // namespace ui
 
 #endif  // SERVICES_UI_WS_WINDOW_TREE_H_