Makes WindowTree::SetFocus(null) work

components/mus/public/cpp/window_tree_connection.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_TREE_CONNECTION_H_
 #define COMPONENTS_MUS_PUBLIC_CPP_WINDOW_TREE_CONNECTION_H_
 
 #include <stdint.h>
 
 #include <map>
@@ skipping 59 matching lines @@
   virtual Window* GetWindowById(Id id) = 0;
 
   // Returns the Window with input capture; null if no window has requested
   // input capture, or if another app has capture.
   virtual Window* GetCaptureWindow() = 0;
 
   // Returns the focused window; null if focus is not yet known or another app
   // is focused.
   virtual Window* GetFocusedWindow() = 0;
 
+  // Sets focus to null. This does nothing if focus is currently null.
+  virtual void ClearFocus() = 0;
+
   // Creates and returns a new Window (which is owned by the window server).
   // Windows are initially hidden, use SetVisible(true) to show.
   Window* NewWindow() { return NewWindow(nullptr); }
   virtual Window* NewWindow(
       const std::map<std::string, std::vector<uint8_t>>* properties) = 0;
   virtual Window* NewTopLevelWindow(
       const std::map<std::string, std::vector<uint8_t>>* properties) = 0;
 
   // Returns the id for this connection.
   // TODO(sky): remove this. It is not necessarily correct anymore.
   virtual ConnectionSpecificId GetConnectionId() = 0;
 
   virtual void AddObserver(WindowTreeConnectionObserver* observer) = 0;
   virtual void RemoveObserver(WindowTreeConnectionObserver* observer) = 0;
 };
 
 }  // namespace mus
 
 #endif  // COMPONENTS_MUS_PUBLIC_CPP_WINDOW_TREE_CONNECTION_H_