Fixes bug in DesktopWindowTreeHostMus's activation handling

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 513 matching lines @@
       ->GetDisplayNearestWindow(const_cast<aura::Window*>(window()))
       .work_area();
 }
 
 void DesktopWindowTreeHostMus::SetShape(
     std::unique_ptr<SkRegion> native_region) {
   NOTIMPLEMENTED();
 }
 
 void DesktopWindowTreeHostMus::Activate() {
+  if (!IsVisible())
+    return;
+
+  // This should result in OnActiveFocusClientChanged() being called, which
+  // triggers a call to DesktopNativeWidgetAura::HandleActivationChanged(),
+  // which focuses the right window.
   aura::Env::GetInstance()->SetActiveFocusClient(
       aura::client::GetFocusClient(window()), window());
-  if (is_active_) {
-    window()->Focus();

[sky, 2017/02/10 19:18:48]

The explicit call to Focus() isn't necessary here as HandleActivationChanged()
(called in OnActiveFocusClientChanged()) focuses the right window.

-    if (window()->GetProperty(aura::client::kDrawAttentionKey))
-      window()->SetProperty(aura::client::kDrawAttentionKey, false);
-  }
+  if (is_active_)
+    window()->SetProperty(aura::client::kDrawAttentionKey, false);
 }
 
 void DesktopWindowTreeHostMus::Deactivate() {
   if (is_active_)
     DeactivateWindow();
 }
 
 bool DesktopWindowTreeHostMus::IsActive() const {
   return is_active_;
 }
@@ skipping 244 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