Make ui::InputDeviceEventObserver provide a default implementation

ui/events/devices/input_device_event_observer.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 UI_EVENTS_DEVICES_INPUT_DEVICE_EVENT_OBSERVER_H_
 #define UI_EVENTS_DEVICES_INPUT_DEVICE_EVENT_OBSERVER_H_
 
 #include "ui/events/devices/events_devices_export.h"
 
 namespace ui {
 
 // DeviceDataManager observer used to announce input hotplug events.
 class EVENTS_DEVICES_EXPORT InputDeviceEventObserver {
  public:
   virtual ~InputDeviceEventObserver() {}
 
-  virtual void OnKeyboardDeviceConfigurationChanged() = 0;
-  virtual void OnTouchscreenDeviceConfigurationChanged() = 0;
-  virtual void OnMouseDeviceConfigurationChanged() = 0;
-  virtual void OnTouchpadDeviceConfigurationChanged() = 0;
+  virtual void OnKeyboardDeviceConfigurationChanged() {}
+  virtual void OnTouchscreenDeviceConfigurationChanged() {}
+  virtual void OnMouseDeviceConfigurationChanged() {}
+  virtual void OnTouchpadDeviceConfigurationChanged() {}
+
+ protected:
+  InputDeviceEventObserver() {}
 };
 
 }  // namespace ui
 
 #endif  // UI_EVENTS_DEVICES_INPUT_DEVICE_EVENT_OBSERVER_H_