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

ui/views/mus/desktop_window_tree_host_mus.cc

 // 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.
 
 #include "ui/views/mus/desktop_window_tree_host_mus.h"
 
 #include "base/memory/ptr_util.h"
 #include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
 #include "base/threading/thread_task_runner_handle.h"
@@ skipping 657 matching lines @@
     return;  // Nothing to do.
 
   wm::SetWindowFullscreen(window(), fullscreen);
 }
 
 bool DesktopWindowTreeHostMus::IsFullscreen() const {
   return window()->GetProperty(aura::client::kShowStateKey) ==
          ui::SHOW_STATE_FULLSCREEN;
 }
 void DesktopWindowTreeHostMus::SetOpacity(float opacity) {
-  // TODO: this likely need to go to server so that non-client decorations get
-  // opacity. http://crbug.com/663619.
-  window()->layer()->SetOpacity(opacity);
+  WindowTreeHostMus::SetOpacity(opacity);
 }
 
 void DesktopWindowTreeHostMus::SetWindowIcons(const gfx::ImageSkia& window_icon,
                                               const gfx::ImageSkia& app_icon) {
   NativeWidgetAura::AssignIconToAuraWindow(window(), window_icon, app_icon);
 }
 
 void DesktopWindowTreeHostMus::InitModalType(ui::ModalType modal_type) {
   window()->SetProperty(aura::client::kModalKey, modal_type);
 }
@@ skipping 107 matching lines @@
     aura::client::FocusClient* focus_client,
     aura::Window* window) {
   if (window == this->window()) {
     desktop_native_widget_aura_->HandleActivationChanged(true);
   } else if (is_active_) {
     desktop_native_widget_aura_->HandleActivationChanged(false);
   }
 }
 
 }  // namespace views