[Mojo Video Capture] Hook up video capture service behind a feature flag

media/capture/video/video_capture_system.h

 // Copyright 2017 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 MEDIA_CAPTURE_VIDEO_VIDEO_CAPTURE_SYSTEM_H_
 #define MEDIA_CAPTURE_VIDEO_VIDEO_CAPTURE_SYSTEM_H_
 
 #include "media/capture/video/video_capture_device_factory.h"
 #include "media/capture/video/video_capture_device_info.h"
 
 namespace media {
 
 // GetDeviceInfosAsync() should be called at least once before calling
 // CreateDevice(), because otherwise CreateDevice() will allways return nullptr.
 class CAPTURE_EXPORT VideoCaptureSystem {
  public:
   using DeviceInfoCallback =
       base::Callback<void(const std::vector<VideoCaptureDeviceInfo>&)>;
 
   virtual ~VideoCaptureSystem() {}
 
   // The passed-in |result_callback| must have ownership of the called
   // VideoCaptureSystem instance to guarantee that it stays alive during the
-  // asynchronous operation.
+  // asynchronous operation. |result_callback| is invoked on the same thread
+  // that calls GetDeviceInfosAsync()
   virtual void GetDeviceInfosAsync(
       const DeviceInfoCallback& result_callback) = 0;
 
   // Creates a VideoCaptureDevice object. Returns nullptr if something goes
   // wrong.
   virtual std::unique_ptr<VideoCaptureDevice> CreateDevice(
       const std::string& device_id) = 0;
 };
 
 }  // namespace media
 
 #endif  // MEDIA_CAPTURE_VIDEO_VIDEO_CAPTURE_SYSTEM_H_