[sensors] Introduce asynchronous way to create sensors.

device/generic_sensor/fake_platform_sensor_provider.h

Index: device/generic_sensor/fake_platform_sensor_provider.h
diff --git a/device/generic_sensor/fake_platform_sensor_provider.h b/device/generic_sensor/fake_platform_sensor_provider.h
index ea2178b4c3f10b6824f2df37f83c9d2d5d2f80bf..551e8210944eddd3eb0206f5a0decf47ef5139e3 100644
--- a/device/generic_sensor/fake_platform_sensor_provider.h
+++ b/device/generic_sensor/fake_platform_sensor_provider.h
@@ -17,10 +17,10 @@ class FakePlatformSensorProvider : public PlatformSensorProvider {
   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);