Makes WindowTree::SetFocus(null) work

components/mus/public/cpp/window.h

 // 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.
 
 #ifndef COMPONENTS_MUS_PUBLIC_CPP_WINDOW_H_
 #define COMPONENTS_MUS_PUBLIC_CPP_WINDOW_H_
 
 #include <stdint.h>
 #include <vector>
 
@@ skipping 178 matching lines @@
 
   Window* GetChildById(Id id);
 
   void SetTextInputState(mojo::TextInputStatePtr state);
   void SetImeVisibility(bool visible, mojo::TextInputStatePtr state);
 
   bool HasCapture() const;
   void SetCapture();
   void ReleaseCapture();
 
-  // Focus.
+  // Focus. See WindowTreeConnection::ClearFocus() to reset focus.
   void SetFocus();
   bool HasFocus() const;
   void SetCanFocus(bool can_focus);
 
   // Embedding. See window_tree.mojom for details.
   void Embed(mus::mojom::WindowTreeClientPtr client);
 
   // NOTE: callback is run synchronously if Embed() is not allowed on this
   // Window.
   void Embed(mus::mojom::WindowTreeClientPtr client,
@@ skipping 121 matching lines @@
   };
 
   std::map<const void*, Value> prop_map_;
 
   DISALLOW_COPY_AND_ASSIGN(Window);
 };
 
 }  // namespace mus
 
 #endif  // COMPONENTS_MUS_PUBLIC_CPP_WINDOW_H_