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

public/platform/WebBatteryStatusListener.h

Index: public/platform/WebBatteryStatusListener.h
diff --git a/public/platform/WebBatteryStatusListener.h b/public/platform/WebBatteryStatusListener.h
index f385b15b8f81de5d404f4ca0fee9805cb5fb5a4e..decb8074194b17ef3d3ce9c5279190a990700e16 100644
--- a/public/platform/WebBatteryStatusListener.h
+++ b/public/platform/WebBatteryStatusListener.h
@@ -6,12 +6,13 @@
 #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;