[Region Capture][cropTo][#5] Introduce cropTo()

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:
--------
* Expose cropTo() on BrowserCaptureMediaStreamTrack.
* Perform some initial sanity checks on the input (must be
  a valid UUID or the empty string).
* Plumb the crop-ID down to the browser process.
* Wire the Promise returned by cropTo() to be resolved
  in response to the callback invocation from the
  browser process.

Next CLs:
---------
Plumb the message down from the browser process to
the GPU process.

Bug: 1247761
Change-Id: I3085a65118ae017ae22ec307c86c2c79062fd49a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3218002
Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: mark a. foltz <mfoltz@chromium.org>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Reviewed-by: Jordan Bayles <jophba@chromium.org>
Commit-Queue: Elad Alon <eladalon@chromium.org>
Cr-Commit-Position: refs/heads/main@{#935451}