Introduce PlatformEvent to handle Device events, Battery events and Gamepad events.

public/platform/WebGamepadListener.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 WebGamepadListener_h
 #define WebGamepadListener_h
 
+#include "WebPlatformEventListener.h"
+
 namespace blink {
 
 class WebGamepad;
 
-class WebGamepadListener {
+class WebGamepadListener : public WebPlatformEventListener {
 public:
     virtual void didConnectGamepad(unsigned index, const blink::WebGamepad&) = 0;
     virtual void didDisconnectGamepad(unsigned index, const blink::WebGamepad&) = 0;
 
 protected:
     virtual ~WebGamepadListener() { }
 };
 
 } // namespace blink
 
 #endif