Implement PollingThread for Device Motion.

content/browser/device_orientation/data_fetcher_shared_memory_default.cc

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "data_fetcher_shared_memory.h"
 
 #include "base/logging.h"
 
 namespace content {
 
 DataFetcherSharedMemory::~DataFetcherSharedMemory() {
-  StopFetchingDeviceMotionData();
+  if (started_)
+    StopFetchingDeviceMotionData();
 }
 
 bool DataFetcherSharedMemory::NeedsPolling() {
   return false;
 }
 
 bool DataFetcherSharedMemory::FetchDeviceMotionDataIntoBuffer() {
   NOTREACHED();
   return false;
 }
 
 bool DataFetcherSharedMemory::StartFetchingDeviceMotionData(
     DeviceMotionHardwareBuffer* buffer) {
   DCHECK(buffer);
+  device_motion_buffer_ = buffer;
   device_motion_buffer_->seqlock.WriteBegin();
   device_motion_buffer_->data.allAvailableSensorsAreActive = true;
   device_motion_buffer_->seqlock.WriteEnd();
+  started_ = true;
   return true;
 }
 
 void DataFetcherSharedMemory::StopFetchingDeviceMotionData() {
   device_motion_buffer_->seqlock.WriteBegin();
   device_motion_buffer_->data.allAvailableSensorsAreActive = false;
   device_motion_buffer_->seqlock.WriteEnd();
+  started_ = false;
 }
 
 }  // namespace content