[Tab Group Parity] Explicitly show dialog for groups of size 1

We missed the case of using the bulk editor and grouping a single tab
should show the dialog.

This was overlooked in:
https://chromium-review.googlesource.com/c/chromium/src/+/5604931

Address this by adding the functionality and tests.

Bug: 338660531
Change-Id: I8f3fab74ed533491328d8b52c29d9e7045885901
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5608254


Commit-Queue: Xi Han <hanxi@chromium.org>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Commit-Queue: Calder Kitagawa <ckitagawa@chromium.org>
Reviewed-by: Xi Han <hanxi@chromium.org>
Auto-Submit: Calder Kitagawa <ckitagawa@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1312009}