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Add the video capture client implementation for the Mirroring service.
It requests to start video capturing through the
media::mojom::VideoCaptureHost interface, and receive the captured
video frames through media::mojom::VideoCaptureObserver interface.

Bug: 734672
Change-Id: I8007d9c8cbca1639eee8c5659f0b34a30c85e375
Reviewed-on: https://chromium-review.googlesource.com/954056Reviewed-by: Yuri Wiitala <miu@chromium.org>
Commit-Queue: Xiangjun Zhang <xjz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543908}