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This change enables CSS and Web animations to apply animated
effects on CSS custom properties.

This change involves a restructure of style resolving detailed
in the design doc:
https://docs.google.com/a/chromium.org/document/d/1V27q30H-pQZVbzHCJjdFThBSeuvxkGLdGmb1hLWBAiY

This is a reland of https://codereview.chromium.org/2309963002
with fixes for when the baseComputedStyle optimisation is active.
The revert was due to crbug.com/669790 where behaviour differs
depending on whether asserts are enabled, thus none of the try
bots picked up on the bug.

BUG=644148

Review-Url: https://codereview.chromium.org/2532953008
Cr-Commit-Position: refs/heads/master@{#436235}