Make ui::InputDeviceEventObserver provide a default implementation

chrome/browser/chromeos/system/pointer_device_observer.h

 // Copyright (c) 2012 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 CHROME_BROWSER_CHROMEOS_SYSTEM_POINTER_DEVICE_OBSERVER_H_
 #define CHROME_BROWSER_CHROMEOS_SYSTEM_POINTER_DEVICE_OBSERVER_H_
 
 #include "base/memory/weak_ptr.h"
 #include "base/observer_list.h"
 #include "chrome/browser/chromeos/device_hierarchy_observer.h"
@@ skipping 26 matching lines @@
   void AddObserver(Observer* observer);
   void RemoveObserver(Observer* observer);
 
  private:
   // DeviceHierarchyObserver:
   void DeviceHierarchyChanged() override;
   void DeviceAdded(int device_id) override {}
   void DeviceRemoved(int device_id) override {}
 
   // InputDeviceEventObserver:
-  void OnTouchscreenDeviceConfigurationChanged() override {}
-  void OnKeyboardDeviceConfigurationChanged() override {}
   void OnMouseDeviceConfigurationChanged() override;
   void OnTouchpadDeviceConfigurationChanged() override;
 
   // Check for pointer devices.
   void CheckTouchpadExists();
   void CheckMouseExists();
 
   // Callback for pointer device checks.
   void OnTouchpadExists(bool exists);
   void OnMouseExists(bool exists);
 
   ObserverList<Observer> observers_;
 
   base::WeakPtrFactory<PointerDeviceObserver> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(PointerDeviceObserver);
 };
 
 }  // namespace system
 }  // namespace chromeos
 
 #endif  // CHROME_BROWSER_CHROMEOS_SYSTEM_POINTER_DEVICE_OBSERVER_H_