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

media/capture/video/video_capture_system.h

Index: media/capture/video/video_capture_system.h
diff --git a/media/capture/video/video_capture_system.h b/media/capture/video/video_capture_system.h
index 3897a20d3902759c0e3e8655c73ccabd0cb8d224..81257fe748621e6048771b25229cb19485b65ac9 100644
--- a/media/capture/video/video_capture_system.h
+++ b/media/capture/video/video_capture_system.h
@@ -21,7 +21,8 @@ class CAPTURE_EXPORT 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;