Minor clean up in tab model classes.

Change-Id: Ib48bba98a4e89ecfb61f48d59fec52d0a4aa01ea
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5381470


Commit-Queue: Sky Malice <skym@chromium.org>
Reviewed-by: David Trainor <dtrainor@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1277009}