Add comment about WSL::GetGroups

The set is in the order of creation of the TabGroups, not the order in
which they appear in the WebStateList.
(We could change this but I don't think there is a need for now.)

Change-Id: Idddf9699670bb369c136a9935ba015e1d3bb8829
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5398100


Commit-Queue: Asami Doi <asamidoi@chromium.org>
Reviewed-by: Aliona Dangla <alionadangla@chromium.org>
Auto-Submit: Louis Romero <lpromero@google.com>
Reviewed-by: Asami Doi <asamidoi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1278854}