[Mojo Video Capture] Use actual factory instances implementing media::VideoCaptureDeviceFactory

services/video_capture/public/interfaces/video_capture_device_factory.mojom

 // 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.
 
 module video_capture.mojom;
 
 import "media/capture/mojo/video_capture_types.mojom";
 import "services/video_capture/public/interfaces/video_capture_device_descriptor.mojom";
 import "services/video_capture/public/interfaces/video_capture_device_proxy.mojom";
 import "services/video_capture/public/interfaces/video_capture_settings.mojom";
@@ skipping 16 matching lines @@
 interface VideoCaptureDeviceFactory {
   EnumerateDeviceDescriptors()
       => (array<VideoCaptureDeviceDescriptor> descriptors);
 
   GetSupportedFormats(VideoCaptureDeviceDescriptor device_descriptor)
       => (array<VideoCaptureFormat> supported_formats);
 
   // Provides exclusive access to the device identified by |device_descriptor|.
   // The access is valid until either the message pipe associated with
   // |proxy_request| is closed by the client, or a subsequent call to
-  // CreateDeviceProxy() is made.
+  // CreateDeviceProxy() is made. When a subsequent call is made while the
+  // |proxy_request| from a previous call has not yet been closed, the service
+  // "revokes" that previous |proxy_request| (by closing the connection) and the
+  // exclusive access goes to the new |proxy_request|.
+  // Note: This design avoids the issue of it not being clear how soon after a
+  // |proxy_request| has been (asynchronously) discarded a subsequent call to
+  // CreateDeviceProxy() would succeed if it were to deny access while still
+  // in use.
   CreateDeviceProxy(VideoCaptureDeviceDescriptor device_descriptor,
                     VideoCaptureDeviceProxy& proxy_request)
       => (DeviceAccessResultCode result_code);
 };