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

ui/aura/mus/window_tree_host_mus.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 UI_AURA_MUS_WINDOW_TREE_HOST_MUS_H_
 #define UI_AURA_MUS_WINDOW_TREE_HOST_MUS_H_
 
 #include <stdint.h>
 
 #include <map>
@@ skipping 55 matching lines @@
   InputMethodMus* input_method() { return input_method_.get(); }
 
   // Sets the client area on the underlying mus window.
   void SetClientArea(const gfx::Insets& insets,
                      const std::vector<gfx::Rect>& additional_client_area);
 
   // Sets the hit test mask on the underlying mus window. Pass base::nullopt to
   // clear.
   void SetHitTestMask(const base::Optional<gfx::Rect>& rect);
 
+  // Sets the opacity of the underlying mus window.
+  void SetOpacity(float value);
+
   // Requests that the window manager change the activation to the next window.
   void DeactivateWindow();
 
   // Requests that our root window be stacked above this other root window
   // which our connection owns.
   void StackAbove(Window* window);
 
   // Requests that our root window be stacked above all other parallel root
   // windows which we might not own.
   void StackAtTop();
@@ skipping 30 matching lines @@
   bool in_set_bounds_from_server_ = false;
 
   std::unique_ptr<InputMethodMus> input_method_;
 
   DISALLOW_COPY_AND_ASSIGN(WindowTreeHostMus);
 };
 
 }  // namespace aura
 
 #endif  // UI_AURA_MUS_WINDOW_TREE_HOST_MUS_H_