Skeleton implementation of CSS Properties and Values API

This patch provides a basic skeleton implementation of the
CSS Properties and Values API. In particular, the pipeline from
registration to reading from computed styles is supported but with
many missing bits along the way.

Some of the missing bits:
- Only * and <length> syntax supported with trivial parsing logic.
- All properties assumed to be inherited.
- Computation logic is currently a no-op (needs to resolve ems, etc.)
- CSSOM on specified styles aren't yet affected by (un)registration

Spec: https://drafts.css-houdini.org/css-properties-values-api/
Design notes: https://docs.google.com/document/d/1IBT3qlZK-BznqcVANckj6kjMjpyFHc-KHdBLz1n4M4c

BUG=641877

Review-Url: https://codereview.chromium.org/2310823002
Cr-Commit-Position: refs/heads/master@{#417240}
38 files changed