[sensors] Introduce asynchronous way to create sensors.

device/generic_sensor/fake_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_FAKE_PLATFORM_SENSOR_PROVIDER_H_
 #define DEVICE_GENERIC_SENSOR_FAKE_PLATFORM_SENSOR_PROVIDER_H_
 
 #include "device/generic_sensor/platform_sensor_provider.h"
 
 namespace device {
 
 class FakePlatformSensorProvider : public PlatformSensorProvider {
  public:
   FakePlatformSensorProvider();
   ~FakePlatformSensorProvider() override;
 
   static FakePlatformSensorProvider* GetInstance();
 
  protected:
-  scoped_refptr<PlatformSensor> CreateSensorInternal(
-      mojom::SensorType type,
-      mojo::ScopedSharedBufferMapping mapping,
-      uint64_t buffer_size) override;
+  void CreateSensorInternal(mojom::SensorType type,
+                            mojo::ScopedSharedBufferMapping mapping,
+                            uint64_t buffer_size,
+                            const CreateSensorCallback& callback) override;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(FakePlatformSensorProvider);
 };
 
 }  // namespace device
 
 #endif  // DEVICE_GENERIC_SENSOR_MOCK_PLATFORM_SENSOR_PROVIDER_H_