Makes WindowTree::SetFocus(null) work

components/mus/public/cpp/lib/window_tree_client_impl.cc

 // Copyright 2014 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 "components/mus/public/cpp/lib/window_tree_client_impl.h"
 
 #include <stddef.h>
 #include <utility>
 
 #include "base/bind.h"
@@ skipping 506 matching lines @@
 
 Window* WindowTreeClientImpl::GetWindowById(Id id) {
   IdToWindowMap::const_iterator it = windows_.find(id);
   return it != windows_.end() ? it->second : NULL;
 }
 
 Window* WindowTreeClientImpl::GetFocusedWindow() {
   return focused_window_;
 }
 
+void WindowTreeClientImpl::ClearFocus() {
+  if (!focused_window_)
+    return;
+
+  SetFocus(nullptr);
+}
+
 Window* WindowTreeClientImpl::NewWindow(
     const Window::SharedProperties* properties) {
   return NewWindowImpl(NewWindowType::CHILD, properties);
 }
 
 Window* WindowTreeClientImpl::NewTopLevelWindow(
     const Window::SharedProperties* properties) {
   return NewWindowImpl(NewWindowType::TOP_LEVEL, properties);
 }
 
@@ skipping 446 matching lines @@
 
 void WindowTreeClientImpl::SetUnderlaySurfaceOffsetAndExtendedHitArea(
     Window* window,
     const gfx::Vector2d& offset,
     const gfx::Insets& hit_area) {
   window_manager_internal_client_->SetUnderlaySurfaceOffsetAndExtendedHitArea(
       window->id(), offset.x(), offset.y(), mojo::Insets::From(hit_area));
 }
 
 }  // namespace mus