[Mojo Video Capture] Introduce abstraction VideoCaptureSystem

media/capture/video/video_capture_device_factory.h

Index: media/capture/video/video_capture_device_factory.h
diff --git a/media/capture/video/video_capture_device_factory.h b/media/capture/video/video_capture_device_factory.h
index 49d9519fee9bbb57d2a3ca84029db4cea2f6a2ff..f0b9b09e413cc4ab61d5f3c8eedb79aeffbc3437 100644
--- a/media/capture/video/video_capture_device_factory.h
+++ b/media/capture/video/video_capture_device_factory.h
@@ -36,6 +36,8 @@ class CAPTURE_EXPORT VideoCaptureDeviceFactory {
       const VideoCaptureDeviceDescriptor& device_descriptor) = 0;
 
   // Asynchronous version of GetDeviceDescriptors calling back to |callback|.
+  // TODO(chfremer): Consider removing this if none of the implementations
+  // overrides it. See crbug.com/708233.
   virtual void EnumerateDeviceDescriptors(
       const base::Callback<
           void(std::unique_ptr<VideoCaptureDeviceDescriptors>)>& callback);