[Mojo Video Capture] Simplify media::VideoCaptureDevice::Client:Buffer to a struct

services/video_capture/buffer_tracker_factory_impl.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 "services/video_capture/buffer_tracker_factory_impl.h"
-
-#include "base/memory/ptr_util.h"
-#include "services/video_capture/mojo_shared_memory_buffer_tracker.h"
-
-namespace video_capture {
-
-std::unique_ptr<media::VideoCaptureBufferTracker>
-BufferTrackerFactoryImpl::CreateTracker(media::VideoPixelStorage storage) {
-  switch (storage) {
-    case media::PIXEL_STORAGE_CPU:
-      return base::MakeUnique<MojoSharedMemoryBufferTracker>();
-  }
-  NOTREACHED();
-  return std::unique_ptr<media::VideoCaptureBufferTracker>();
-}
-
-}  // namespace video_capture