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

public/platform/WebGamepadListener.h

Index: public/platform/WebGamepadListener.h
diff --git a/public/platform/WebGamepadListener.h b/public/platform/WebGamepadListener.h
index f73049e461268542a1f2c1b31be17f2d1889a2f5..e292c0a21e10b3f492bd8edf06d065a943824340 100644
--- a/public/platform/WebGamepadListener.h
+++ b/public/platform/WebGamepadListener.h
@@ -5,11 +5,13 @@
 #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;