aura-mus: Implement DesktopWindowTreeHostMus::SetOpacity().

ui/aura/mus/window_tree_client.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 UI_AURA_MUS_WINDOW_TREE_CLIENT_H_
 #define UI_AURA_MUS_WINDOW_TREE_CLIENT_H_
 
 #include <stdint.h>
 
 #include <map>
@@ skipping 416 matching lines @@
   // Overriden from WindowTreeHostMusDelegate:
   void OnWindowTreeHostBoundsWillChange(WindowTreeHostMus* window_tree_host,
                                         const gfx::Rect& bounds) override;
   void OnWindowTreeHostClientAreaWillChange(
       WindowTreeHostMus* window_tree_host,
       const gfx::Insets& client_area,
       const std::vector<gfx::Rect>& additional_client_areas) override;
   void OnWindowTreeHostHitTestMaskWillChange(
       WindowTreeHostMus* window_tree_host,
       const base::Optional<gfx::Rect>& mask_rect) override;
+  void OnWindowTreeHostSetOpacity(WindowTreeHostMus* window_tree_host,
+                                  float opacity) override;
   void OnWindowTreeHostDeactivateWindow(
       WindowTreeHostMus* window_tree_host) override;
   void OnWindowTreeHostStackAbove(WindowTreeHostMus* window_tree_host,
                                   Window* window) override;
   void OnWindowTreeHostStackAtTop(WindowTreeHostMus* window_tree_host) override;
   void OnWindowTreeHostPerformWindowMove(
       WindowTreeHostMus* window_tree_host,
       ui::mojom::MoveLoopSource mus_source,
       const gfx::Point& cursor_location,
       const base::Callback<void(bool)>& callback) override;
@@ skipping 97 matching lines @@
   std::unique_ptr<ui::Gpu> gpu_;
   std::unique_ptr<MusContextFactory> compositor_context_factory_;
   base::WeakPtrFactory<WindowTreeClient> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(WindowTreeClient);
 };
 
 }  // namespace aura
 
 #endif  // UI_AURA_MUS_WINDOW_TREE_CLIENT_H_