[sensors] Introduce asynchronous way to create sensors.

device/generic_sensor/fake_platform_sensor_provider.cc

 // 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.
 
 #include "base/memory/singleton.h"
 
 #include "device/generic_sensor/fake_platform_sensor.h"
 #include "device/generic_sensor/fake_platform_sensor_provider.h"
 
 namespace device {
 
 // static
 FakePlatformSensorProvider* FakePlatformSensorProvider::GetInstance() {
   return base::Singleton<
       FakePlatformSensorProvider,
       base::LeakySingletonTraits<FakePlatformSensorProvider>>::get();
 }
 
 FakePlatformSensorProvider::FakePlatformSensorProvider() = default;
 
 FakePlatformSensorProvider::~FakePlatformSensorProvider() = default;
 
-scoped_refptr<PlatformSensor> FakePlatformSensorProvider::CreateSensorInternal(
+void FakePlatformSensorProvider::CreateSensorInternal(
     mojom::SensorType type,
     mojo::ScopedSharedBufferMapping mapping,
-    uint64_t buffer_size) {
-  return new FakePlatformSensor(type, std::move(mapping), buffer_size, this);
+    uint64_t buffer_size,
+    const CreateSensorCallback& callback) {
+  scoped_refptr<FakePlatformSensor> sensor =
+      new FakePlatformSensor(type, std::move(mapping), buffer_size, this);
+  callback.Run(std::move(sensor));
 }
 
 }  // namespace device