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

services/ui/public/cpp/tests/test_window_tree.cc

 // 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.
 
 #include "services/ui/public/cpp/tests/test_window_tree.h"
 
 #include "testing/gtest/include/gtest/gtest.h"
 
 namespace ui {
 
@@ skipping 37 matching lines @@
                                      const gfx::Rect& bounds) {
   got_change_ = true;
   change_id_ = change_id;
 }
 
 void TestWindowTree::SetClientArea(
     uint32_t window_id,
     const gfx::Insets& insets,
     mojo::Array<gfx::Rect> additional_client_areas) {}
 
-void TestWindowTree::SetHitTestMask(uint32_t window_id, const gfx::Rect& mask) {
-}
+void TestWindowTree::SetHitTestMask(uint32_t window_id,
+                                    const base::Optional<gfx::Rect>& mask) {}
 
 void TestWindowTree::SetWindowVisibility(uint32_t change_id,
                                          uint32_t window_id,
                                          bool visible) {
   got_change_ = true;
   change_id_ = change_id;
 }
 
 void TestWindowTree::SetWindowProperty(uint32_t change_id,
                                        uint32_t window_id,
@@ skipping 98 matching lines @@
 }
 
 void TestWindowTree::PerformWindowMove(uint32_t change_id,
                                        uint32_t window_id,
                                        mojom::MoveLoopSource source,
                                        const gfx::Point& cursor_location) {}
 
 void TestWindowTree::CancelWindowMove(uint32_t window_id) {}
 
 }  // namespace ui