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

public/platform/WebBatteryStatusListener.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 WebBatteryStatusListener_h
 #define WebBatteryStatusListener_h
 
 #include "WebBatteryStatus.h"
+#include "WebPlatformEventListener.h"
 
 namespace blink {
 
 class WebBatteryStatus;
 
-class WebBatteryStatusListener {
+class WebBatteryStatusListener : public WebPlatformEventListener {
 public:
     // This method is called when a new battery status is available.
     virtual void updateBatteryStatus(const WebBatteryStatus&) = 0;
 
     virtual ~WebBatteryStatusListener() { }
 };
 
 } // namespace blink
 
 #endif // WebBatteryStatusListener_h