Reland "Delete class WebRtcVideoCapturerAdapter."

This is a reland of bd70ec8c7cf5884fbf517d6b3122b6c29a3a2b9b

Original change's description:
> 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}

Tbr: sergeyu@chromium.org
Bug: webrtc:6353
Change-Id: I828b0ac4842b36dd543ff574cad5eb47be6a4d48
Reviewed-on: https://chromium-review.googlesource.com/c/1382231
Commit-Queue: Niels Möller <nisse@chromium.org>
Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Cr-Commit-Position: refs/heads/master@{#617464}
11 files changed