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This allows us to remove some duplicate code, and prepares the way
for generating ComputedStyle's Initial* functions elsewhere.

This ended up reducing the number of includes in each APO subclass
header. Gist diff:
https://gist.github.com/wilddamon/5bfa127e71e9c7993104bbfa602b3f97/revisions

Bug: 763306
Change-Id: Icaacee6abe218b73a460311eb1ffcbff5c81d4c5
Reviewed-on: https://chromium-review.googlesource.com/716442
Commit-Queue: meade_UTC10 <meade@chromium.org>
Reviewed-by: Darren Shen <shend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#509698}