[css-typed-om] Fix CSSVariableReferenceValue validation.

We change CSSVariableReferenceValue to reject invalid custom property
names as per spec. We also remove .tentative extension from related
tests.

We also remove some unit tests that are replaced by WPTs.

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

Bug: 545318
Change-Id: I95cfe1c167a60c94083a1474b384fd584d05cca9
Reviewed-on: https://chromium-review.googlesource.com/895124
Commit-Queue: Darren Shen <shend@chromium.org>
Reviewed-by: nainar <nainar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#533228}
11 files changed