[SxS] Implement min active width for splits

Implications:
- When there are lots of tabs, active split tabs will be large enough
that the Xs don't feel cramped
- Split tabs will shrink more slowly towards the crossover point

We are still having discussions about whether we want split tabs to
be half the width of normal tabs for as long as possible but
regardless of what we decide to do there, we will need to add this
min active width so split tabs aren't squished around the crossover
point.

Bug: 414921009
Change-Id: I0e1da772c3a8ea59403480b2d67fecf6dcaf311e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6542249
Reviewed-by: Eshwar Stalin <estalin@chromium.org>
Commit-Queue: Alison Gale <agale@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1460080}