[Layout] Adjust comments on layout managers

Adjust comments to not use "LayoutManager" since we technically want
all layout managers used to extend from LayoutManagerBase. Our
reustable layout managers like flexlayout, boxlayout etc already do.

Bug: None
Change-Id: I857748d5f63220681e65382ffcc0f358f7d2592e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5386214


Auto-Submit: Elaine Chien <elainechien@chromium.org>
Commit-Queue: Dana Fried <dfried@chromium.org>
Reviewed-by: Dana Fried <dfried@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1277303}