[sensors](Linux) Fix tsan data race in sensor reader

device/generic_sensor/platform_sensor_linux.h

 // Copyright 2016 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.
 
 #ifndef DEVICE_GENERIC_SENSOR_PLATFORM_SENSOR_LINUX_H_
 #define DEVICE_GENERIC_SENSOR_PLATFORM_SENSOR_LINUX_H_
 
 #include "device/generic_sensor/platform_sensor.h"
 
 namespace base {
@@ skipping 27 matching lines @@
   bool StartSensor(const PlatformSensorConfiguration& configuration) override;
   void StopSensor() override;
   bool CheckSensorConfiguration(
       const PlatformSensorConfiguration& configuration) override;
   PlatformSensorConfiguration GetDefaultConfiguration() override;
 
  private:
   const PlatformSensorConfiguration default_configuration_;
   const mojom::ReportingMode reporting_mode_;
 
+  scoped_refptr<base::SingleThreadTaskRunner> polling_thread_task_runner_;
+
   // A sensor reader that reads values from sensor files
   // and stores them to a SensorReading structure.
   std::unique_ptr<SensorReader> sensor_reader_;
 
   // Stores previously read values that are used to
   // determine whether the recent values are changed
   // and IPC can be notified that updates are available.
   SensorReading old_values_;
 
   base::WeakPtrFactory<PlatformSensorLinux> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(PlatformSensorLinux);
 };
 
 }  // namespace device
 
 #endif  // DEVICE_GENERIC_SENSOR_PLATFORM_SENSOR_LINUX_H_