[Region Capture][cropTo][#6] Plumb crop message to Viz

Overall feature:
----------------
Region Capture is a feature allowing the cropping of self-capture video
tracks to track the coordinates of an HTMLElement in the browsing
context.

Public design-doc:
https://docs.google.com/document/d/1dULARMnMZggfWqa_Ti_GrINRNYXGIli3XK9brzAKEV8/edit?usp=sharing

Spec draft (very WIP): https://eladalon1983.github.io/region-capture/
(This will be finished soon and moved to the WICG; hopefully to be
eventually adopted by the W3C.)

This CL sub-chain:
------------------
The Region Capture feature introduces two new APIs:
* navigator.mediaDevices.produceCropId()
* BrowserCaptureMediaStreamTrack.cropTo()
This CL sub-chain deals with the latter.

This CL:
--------
Plumb the crop message (and callback) all the
way to FrameSinkVideoCaptureDevice.

Next CLs:
---------
* Invoke the callback with a rejection message if not self-capture.
* Send the message onwards to viz::ClientFrameSinkVideoCapturer.

Bug: 1247761
Change-Id: Id86be452ab19aad39ba71a622cd9a689be481d84
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3232118
Auto-Submit: Elad Alon <eladalon@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Jordan Bayles <jophba@chromium.org>
Reviewed-by: mark a. foltz <mfoltz@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Elad Alon <eladalon@chromium.org>
Cr-Commit-Position: refs/heads/main@{#936048}