[TabGroupDragDrop] Add trailing margin to group title

- This also fixes that tab cannot be dropped at the first position in an expanded group.
- The drop position is inaccurate, see demo(14s), plan to fix that in follow-up.

Demo: https://drive.google.com/file/d/1xbZWNr1v2IQl2c729IhqmnO48x7bYjq5/view?usp=sharing

Bug: 409161512
Change-Id: If568a6f3d6b53e082d6e015d64f280d5e9362cd0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6438697
Reviewed-by: Neil Coronado <nemco@google.com>
Commit-Queue: Zhe Li <zheliooo@google.com>
Cr-Commit-Position: refs/heads/main@{#1444928}