CSS Properties and Values API: Implement computation / computational independence

This patch implements basic computation and computational independence
logic for registered custom properties. For now we just handle variable
references, length types and lists thereof, while images and transform
functions containing lengths will be properly handled later.

'Computationally independent' has recently been renamed from
'computationally idempotent', so the function is updated to match.

https://drafts.css-houdini.org/css-properties-values-api/#calculation-of-computed-values
https://drafts.css-houdini.org/css-properties-values-api/#computationally-independent

BUG=641877

Review-Url: https://codereview.chromium.org/2354463002
Cr-Commit-Position: refs/heads/master@{#420328}
7 files changed