[sensors] Add a browser test to sanity check ambient light sensor.

device/generic_sensor/platform_sensor_provider.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_PROVIDER_H_
 #define DEVICE_GENERIC_SENSOR_PLATFORM_SENSOR_PROVIDER_H_
 
 #include "device/generic_sensor/generic_sensor_export.h"
 #include "device/generic_sensor/platform_sensor_provider_base.h"
 
 namespace device {
 
 // This a singleton class returning the actual sensor provider
 // implementation for the current platform.
 class DEVICE_GENERIC_SENSOR_EXPORT PlatformSensorProvider
     : public PlatformSensorProviderBase {
  public:
   // Returns the PlatformSensorProvider singleton.
   // Note: returns 'nullptr' if there is no available implementation for
   // the current platform.
   static PlatformSensorProvider* GetInstance();
 
+  static void SetProviderForTesting(PlatformSensorProvider* provider);

[Mikhail, 2016/10/24 06:41:56]

nit: a short description is welcome

+
  protected:
   PlatformSensorProvider() = default;
   ~PlatformSensorProvider() override = default;
 
   DISALLOW_COPY_AND_ASSIGN(PlatformSensorProvider);
 };
 
 }  // namespace device
 
 #endif  // DEVICE_GENERIC_SENSOR_PLATFORM_SENSOR_PROVIDER_H_