SetIsEnsuredOutsideFlatTree via StyleRecalcContext

The current requirements for setting this flag don't entirely make
sense: whether or not inheritance is allowed is not (directly) related
to whether or not something is in the flat tree.

Make this more explicit by sending our flat tree status via
StyleRecalcContext.

There should be no behavior change.

Change-Id: I5729ce1b9ddb1f1c8b8c1fdf16783571cc2f422d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4667279
Commit-Queue: Anders Hartvoll Ruud <andruud@chromium.org>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1167270}