CSS: Centralize logic for implicit shadow combinators

Refactor to combine NeedsImplicitShadowCombinatorForMatching and the
selection of which new relation type. This is done to centralize these
2 related things and also because ::part will add another case to what
was previously a ternary expression.


Bug: 805271
Change-Id: I2df655c57b952307f64b021b547319849e5356d2
Reviewed-on: https://chromium-review.googlesource.com/970626
Commit-Queue: Fergal Daly <fergal@chromium.org>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#544649}
3 files changed