[Tab Groups] Ensure reserved ID groups are not already in use.

The ID 2001 was already in use by the SendTabToSelfMenu.

Also delete some unused constexprs.

Bug: 1141858
Change-Id: I10f04c0eb01f17d26f4962b37789588adde3e079
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2495713


Reviewed-by: Peter Boström <pbos@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Charlene Yan <cyan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#822913}