Delete class WebRtcVideoCapturerAdapter.

Move functionality into MediaStreamVideoWebRtcSink::WebRtcVideoSource,
inheriting the convenience base class rtc::AdaptedVideoTrackSource.

Bug: webrtc:6353
Change-Id: Ia0eb3bc9addd888eab58981e84378e4cb9da8b5a
Reviewed-on: https://chromium-review.googlesource.com/c/1373757
Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
Commit-Queue: Niels Möller <nisse@chromium.org>
Cr-Commit-Position: refs/heads/master@{#617069}
11 files changed