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

ui/views/mus/desktop_window_tree_host_mus_unittest.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/debug/stack_trace.h"
 
 #include "base/memory/ptr_util.h"
 #include "ui/aura/client/cursor_client.h"
@@ skipping 250 matching lines @@
   std::unique_ptr<Widget> widget2(CreateWidget(nullptr));
   widget2->Show();
 
   aura::test::ChangeCompletionWaiter waiter(
       MusClient::Get()->window_tree_client(),
       aura::ChangeType::REORDER, true);
   widget1->StackAboveWidget(widget2.get());
   waiter.Wait();
 }
 
+TEST_F(DesktopWindowTreeHostMusTest, SetOpacity) {
+  std::unique_ptr<Widget> widget1(CreateWidget(nullptr));
+  widget1->Show();
+
+  aura::test::ChangeCompletionWaiter waiter(
+      MusClient::Get()->window_tree_client(), aura::ChangeType::OPACITY, true);
+  widget1->SetOpacity(0.5f);
+  waiter.Wait();
+}
+
 TEST_F(DesktopWindowTreeHostMusTest, TransientParentWiredToHostWindow) {
   std::unique_ptr<Widget> widget1(CreateWidget());
   widget1->Show();
 
   std::unique_ptr<Widget> widget2(
       CreateWidget(nullptr, widget1->GetNativeView()));
   widget2->Show();
 
   aura::client::TransientWindowClient* transient_window_client =
       aura::client::GetTransientWindowClient();
   // Even though the widget1->GetNativeView() was specified as the parent we
   // expect the transient parents to be marked at the host level.
   EXPECT_EQ(widget1->GetNativeView()->GetHost()->window(),
             transient_window_client->GetTransientParent(
                 widget2->GetNativeView()->GetHost()->window()));
 }
 
 }  // namespace views