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

ui/aura/mus/window_tree_host_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_AURA_MUS_WINDOW_TREE_HOST_MUS_DELEGATE_H_
 #define UI_AURA_MUS_WINDOW_TREE_HOST_MUS_DELEGATE_H_
 
 #include <stdint.h>
 
 #include <map>
@@ skipping 24 matching lines @@
   virtual void OnWindowTreeHostClientAreaWillChange(
       WindowTreeHostMus* window_tree_host,
       const gfx::Insets& client_area,
       const std::vector<gfx::Rect>& additional_client_areas) = 0;
 
   // Called when the hit test mask is about to be cleared.
   virtual void OnWindowTreeHostHitTestMaskWillChange(
       WindowTreeHostMus* window_tree_host,
       const base::Optional<gfx::Rect>& mask_rect) = 0;
 
+  // Called when the opacity is changed client side.
+  virtual void OnWindowTreeHostSetOpacity(WindowTreeHostMus* window_tree_host,
+                                          float opacity) = 0;
+
   // Called to clear the focus of the current window.
   virtual void OnWindowTreeHostDeactivateWindow(
       WindowTreeHostMus* window_tree_host) = 0;
 
   // Called to stack the native window above the native window of |window|.
   virtual void OnWindowTreeHostStackAbove(
       WindowTreeHostMus* window_tree_host,
       Window* window) = 0;
 
   // Called to stack the native window above other native windows.
@@ skipping 21 matching lines @@
   // created.
   virtual void OnWindowTreeHostCreated(WindowTreeHostMus* window_tree_host) = 0;
 
  protected:
   virtual ~WindowTreeHostMusDelegate() {}
 };
 
 }  // namespace aura
 
 #endif  // UI_AURA_MUS_WINDOW_TREE_HOST_MUS_DELEGATE_H_