[Region Capture #2] Add region_capture_token to capturer

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:
--------
This patch is the second in the series of patches for region capture
supporting tab capture. A new API is added to the video capturer to
allow setting a crop id (currently a base::Token) associated
with a set of region capture bounds, as well as a new API allowing
setting region capture bounds on the compositor render pass.

Next CLs:
---------
From here, the compositor render pass bounds will have to be set in the
compositor and paint system to connect element rare data with the video
capturer.

Bug: 1247761
Change-Id: Ie6b9fae888a3a71fb7ad22eb20942958a29eed86
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3180067
Commit-Queue: Jordan Bayles <jophba@chromium.org>
Reviewed-by: Robert Kroeger <rjkroege@chromium.org>
Reviewed-by: Elad Alon <eladalon@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Reviewed-by: Maggie Chen <magchen@chromium.org>
Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Reviewed-by: mark a. foltz <mfoltz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#931724}