VT: Update the API to the latest spec

This patch changes the API to the new iteration that's in the spec.
The patch is pretty large! Here's a brief guide:
1. Most of the API and model changes are in document_transition.cc
   This is a big one that should be reviewed carefully
2. Since we no longer reuse a transition, and a transition object
   does not necessarily exist, supplement now has more
   responsibility about tracking what's created and what's not
3. Due to above, the rest of the changes are plumbing changes that use
   DocumentTransitionUtils to get the transition if one exists instead
   of getting the supplement directly
4. The test changes are just updating to the new API (for the most part)
5. Removed the stop transition API since it was a bit of a wart for
   testing purposes, and we have other test coverage in the area
6. Removed some tests that don't make sense in the new API.

R=khushalsagar@chromium.org, bokan@chromium.org

Change-Id: I015c7defa82c03e984b273c400b0ca777b58d4f4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3957574
Reviewed-by: Mike West <mkwst@chromium.org>
Reviewed-by: Khushal Sagar <khushalsagar@chromium.org>
Reviewed-by: David Bokan <bokan@chromium.org>
Commit-Queue: Vladimir Levin <vmpstr@chromium.org>
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1062134}