Refactor CSSComputedStyleDeclaration's style/layout updates

Proper style and layout updates are currently strongly tied to
CSSComputedStyleDeclaration::GetPropertyCSSValue. In the future, we'll
need to be style (and potentially layout) clean in other function as
well. Hence, split out the relevant code into utils.

There should be no behavior change.

Bug: 949807
Change-Id: Ie9278776e3cee97aa72d43ba54a14b3cd14341a7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2821806


Commit-Queue: Anders Hartvoll Ruud <andruud@chromium.org>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#872378}