[Video Capture Service] Fix for VideoFacing constraint not working

The struct media::VideoCaptureDeviceDescriptor contains a field for facing_mode.
The corresponding mojo struct does not contain this field. Because of it, the
information will be dropped when crossing a Mojo boundary, i.e. when the video
capture service is used.

Bug: 829114
Change-Id: I8193c3db125450e80dd9f30b7babee3063013ef9
Reviewed-on: https://chromium-review.googlesource.com/1140956
Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Christian Fremerey <chfremer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575840}
3 files changed