[TabGroupIndicators] Hide dividers adjacent to group titles

Hide dividers adjacent to group titles.

Screenshot: https://screenshot.googleplex.com/BL57zi9XFsNSxU6.png

Bug: 328503577
Change-Id: I7f90cd6761c37ea362bacc409947dac6888a2f94
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5350396


Commit-Queue: Neil Coronado <nemco@google.com>
Reviewed-by: Zhe Li <zheliooo@google.com>
Cr-Commit-Position: refs/heads/main@{#1280297}