Fire null Device Motion events only once.

Source/modules/device_orientation/DeviceOrientationDispatcher.cpp

Index: Source/modules/device_orientation/DeviceOrientationDispatcher.cpp
diff --git a/Source/modules/device_orientation/DeviceOrientationDispatcher.cpp b/Source/modules/device_orientation/DeviceOrientationDispatcher.cpp
index b6989e95340be9c2898e92299d30ee579cfe7a56..b4836ed06f681216fa7a5bc7027bf8eb429b7623 100644
--- a/Source/modules/device_orientation/DeviceOrientationDispatcher.cpp
+++ b/Source/modules/device_orientation/DeviceOrientationDispatcher.cpp
@@ -69,20 +69,23 @@ void DeviceOrientationDispatcher::startListening()
 void DeviceOrientationDispatcher::stopListening()
 {
     WebKit::Platform::current()->setDeviceOrientationListener(0);
+    m_lastDeviceOrientationData.clear();
 }
 
 void DeviceOrientationDispatcher::didChangeDeviceOrientation(const WebKit::WebDeviceOrientationData& motion)
 {
     m_lastDeviceOrientationData = DeviceOrientationData::create(motion);
-    bool needsPurge = false;
-    for (size_t i = 0; i < m_controllers.size(); ++i) {
+
+    m_isDispatching = true;
+    // Don't fire controllers removed or added during event dispatch.
+    size_t end = m_controllers.size();
+    for (size_t i = 0; i < end; ++i) {
         if (m_controllers[i])
             static_cast<NewDeviceOrientationController*>(m_controllers[i])->didChangeDeviceOrientation(m_lastDeviceOrientationData.get());
-        else
-            needsPurge = true;
     }
+    m_isDispatching = false;
 
-    if (needsPurge)
+    if (m_needsPurge)
         purgeControllers();
 }