[Typed-OM] Add CSSVariableReferenceValue

Spec: https://drafts.css-houdini.org/css-typed-om/#cssvariablereferencevalue

BUG=545318

Review-Url: https://codereview.chromium.org/2126713002
Cr-Commit-Position: refs/heads/master@{#404937}
6 files changed