Cleanup: Add more conversion helpers for usage.

Source/modules/device_orientation/DeviceMotionController.cpp

 /*
  * Copyright 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2012 Samsung Electronics. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  *  * Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  *  * Redistributions in binary form must reproduce the above copyright
@@ skipping 73 matching lines @@
     DeviceMotionDispatcher::instance().addDeviceMotionController(this);
 }
 
 void DeviceMotionController::unregisterWithDispatcher()
 {
     DeviceMotionDispatcher::instance().removeDeviceMotionController(this);
 }
 
 bool DeviceMotionController::isNullEvent(Event* event)
 {
-    ASSERT(event->type() == EventNames::devicemotion);
-    DeviceMotionEvent* motionEvent = static_cast<DeviceMotionEvent*>(event);
+    DeviceMotionEvent* motionEvent = toDeviceMotionEvent(event);
     return !motionEvent->deviceMotionData()->canProvideEventData();
 }
 
 void DeviceMotionController::didAddEventListener(DOMWindow*, const AtomicString& eventType)
 {
     if (eventType == EventNames::devicemotion && RuntimeEnabledFeatures::deviceMotionEnabled()) {
         if (page() && page()->visibilityState() == PageVisibilityStateVisible)
             startUpdating();
         m_hasEventListener = true;
     }
@@ skipping 18 matching lines @@
     if (!m_hasEventListener)
         return;
 
     if (page()->visibilityState() == PageVisibilityStateVisible)
         startUpdating();
     else
         stopUpdating();
 }
 
 } // namespace WebCore