Correctly cascade before-change style for @starting-style

Before-change style should inherit from parent after-change style for
@starting-style. That means we need to cascade after-change separately
also for before-change style in this case.

This CL separates out ensuring of after-change style so that it can be
shared between before- and after-change style computations.

Also, the StyleRecalcContext must be set up correctly with old_style to
ensure that:

1. @starting-style rules are not applied for after-change style.
2. @starting-style rules are applied for before-change style when
   @starting-style is the before-change for a given style resolution.

Bug: 40337057

Change-Id: I92af1934fbab45bb294ea92055ec3be373a8caa5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6185041
Reviewed-by: Anders Hartvoll Ruud <andruud@chromium.org>
Commit-Queue: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1409627}