[grid] Unify "2nd passes" within grid layout.

Previously we had two "2nd passes" an "inner" and "outer". This
combines them!

I suspect that we were able to do this after due to other changes we
made.

Change-Id: I35c9d66054f3593de7251eb0c935303c59274059
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3863726


Commit-Queue: Ian Kilpatrick <ikilpatrick@chromium.org>
Reviewed-by: Ethan Jimenez <ethavar@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#1045334}