Fire null Device Motion events only once.

Source/modules/device_orientation/DeviceMotionData.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:
  *  * 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
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 91 matching lines @@
 DeviceMotionData::DeviceMotionData(PassRefPtr<Acceleration> acceleration, PassRefPtr<Acceleration> accelerationIncludingGravity,
                                    PassRefPtr<RotationRate> rotationRate, bool canProvideInterval, double interval)
     : m_acceleration(acceleration)
     , m_accelerationIncludingGravity(accelerationIncludingGravity)
     , m_rotationRate(rotationRate)
     , m_canProvideInterval(canProvideInterval)
     , m_interval(interval)
 {
 }
 
+bool DeviceMotionData::canProvideEventData() const
+{
+    const bool hasAcceleration = m_acceleration && (m_acceleration->canProvideX() || m_acceleration->canProvideY() || m_acceleration->canProvideZ());
+    const bool hasAccelerationIncludingGravity = m_accelerationIncludingGravity && (m_accelerationIncludingGravity->canProvideX() || m_accelerationIncludingGravity->canProvideY() || m_accelerationIncludingGravity->canProvideZ());
+    const bool hasRotationRate = m_rotationRate && (m_rotationRate->canProvideAlpha() || m_rotationRate->canProvideBeta() || m_rotationRate->canProvideGamma());
+
+    return hasAcceleration || hasAccelerationIncludingGravity || hasRotationRate;
+}
+
 } // namespace WebCore