Update DeviceMotionEvent IDL to match the specification.

third_party/WebKit/Source/modules/device_orientation/DeviceMotionEvent.cpp

 /*
  * Copyright (C) 2010 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 62 matching lines @@
   if (!m_deviceMotionData->getRotationRate())
     return nullptr;
 
   if (!m_rotationRate)
     m_rotationRate =
         DeviceRotationRate::create(m_deviceMotionData->getRotationRate());
 
   return m_rotationRate.get();
 }
 
-double DeviceMotionEvent::interval(bool& isNull) const {
-  isNull = false;
+double DeviceMotionEvent::interval() const {
   return m_deviceMotionData->interval();
 }
 
 const AtomicString& DeviceMotionEvent::interfaceName() const {
   return EventNames::DeviceMotionEvent;
 }
 
 DEFINE_TRACE(DeviceMotionEvent) {
   visitor->trace(m_deviceMotionData);
   visitor->trace(m_acceleration);
   visitor->trace(m_accelerationIncludingGravity);
   visitor->trace(m_rotationRate);
   Event::trace(visitor);
 }
 
 }  // namespace blink