[css-typed-om] Update CSSVariableReferenceValue.

The new spec made attributes of CSSVariableReferenceValue mutable.
This also allows us to simplify the test helper to create
CSSVariableReferenceValues.

Spec: https://drafts.css-houdini.org/css-typed-om-1/#cssvariablereferencevalue

Bug: 545318
Change-Id: I9f7571e7b50b646423e4a6dc44cfb6c95c3fc5dd
Reviewed-on: https://chromium-review.googlesource.com/802664
Commit-Queue: Darren Shen <shend@chromium.org>
Reviewed-by: Renée Wright <rjwright@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521936}
7 files changed