Use enum for comparing active and inactive tab width

This avoids an issue where we allocate extra pixels to some tabs and tab
closing mode handles the logic differently than if there aren't extra
pixels.

There are still other uses of the exact values which I'll audit
separately but will need to investigate more to ensure they are safe.

#top-chrome-bug-fixit

Bug: 418256399
Change-Id: I457e9a0cbeb510ad0e5d15cdeeac240e1360a1a9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6557846
Reviewed-by: David Pennington <dpenning@chromium.org>
Commit-Queue: Alison Gale <agale@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1461606}