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

public/platform/WebDeviceMotionListener.h

Index: public/platform/WebDeviceMotionListener.h
diff --git a/public/platform/WebDeviceMotionListener.h b/public/platform/WebDeviceMotionListener.h
index 820271c07ad0780157ead97cdd16e5dba2ab1403..6b49169c18e8b338f1da50fae50f385ac1635bdc 100644
--- a/public/platform/WebDeviceMotionListener.h
+++ b/public/platform/WebDeviceMotionListener.h
@@ -31,11 +31,13 @@
 #ifndef WebDeviceMotionListener_h
 #define WebDeviceMotionListener_h
 
+#include "WebPlatformEventListener.h"
+
 namespace blink {
 
 class WebDeviceMotionData;
 
-class WebDeviceMotionListener {
+class WebDeviceMotionListener : public WebPlatformEventListener {
 public:
     // This method is called every time new device motion data is available.
     virtual void didChangeDeviceMotion(const WebDeviceMotionData&) = 0;