[sensors] Introduce asynchronous way to create sensors.

device/generic_sensor/sensor_provider_impl.h

Index: device/generic_sensor/sensor_provider_impl.h
diff --git a/device/generic_sensor/sensor_provider_impl.h b/device/generic_sensor/sensor_provider_impl.h
index b46d59ac5224f0579408b1e3e2a046ed9dbdaae9..4d8541184fe469d2b4c44d3d0e33d37210ff7fe6 100644
--- a/device/generic_sensor/sensor_provider_impl.h
+++ b/device/generic_sensor/sensor_provider_impl.h
@@ -11,6 +11,7 @@
 namespace device {
 
 class PlatformSensorProvider;
+class PlatformSensor;
 
 // Implementation of SensorProvider mojo interface.
 // Uses PlatformSensorProvider singleton to create platform specific instances
@@ -29,7 +30,15 @@ class SensorProviderImpl final : public mojom::SensorProvider {
                  mojom::SensorRequest sensor_request,
                  const GetSensorCallback& callback) override;
 
+  // Helper callback method to return created sensors.
+  void SensorCreated(mojom::SensorType type,
+                     mojo::ScopedSharedBufferHandle cloned_handle,
+                     mojom::SensorRequest sensor_request,
+                     const GetSensorCallback& callback,
+                     scoped_refptr<PlatformSensor> sensor);
+
   PlatformSensorProvider* provider_;
+  base::WeakPtrFactory<SensorProviderImpl> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(SensorProviderImpl);
 };