[SxS] Clean up tab path logic for side by side

Previously the tabs still had their border radii so they had to overlap
a lot in order to appear as a single tab. With this change the edges of
the two tabs are flush.

- Remove the border radius for FILL paths internal to the split
- Add extra safety to helpers for start/end split tabs

Change-Id: I48e469605198f15e3fe765aa4f0f9727319cfa19
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6453921
Reviewed-by: Shibalik Mohapatra <shibalik@chromium.org>
Commit-Queue: Alison Gale <agale@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1447834}