Make ui::InputDeviceEventObserver provide a default implementation

ash/display/display_change_observer_chromeos.cc

 // Copyright 2013 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 "ash/display/display_change_observer_chromeos.h"
 
 #include <algorithm>
 #include <map>
 #include <set>
 #include <string>
@@ skipping 269 matching lines @@
       return kThresholdTable[i].device_scale_factor;
   }
   return 1.0f;
 }
 
 void DisplayChangeObserver::OnTouchscreenDeviceConfigurationChanged() {
   OnDisplayModeChanged(
       Shell::GetInstance()->display_configurator()->cached_displays());
 }
 
-void DisplayChangeObserver::OnKeyboardDeviceConfigurationChanged() {
-}
-
-void DisplayChangeObserver::OnMouseDeviceConfigurationChanged() {
-}
-
-void DisplayChangeObserver::OnTouchpadDeviceConfigurationChanged() {
-}
-
 }  // namespace ash