The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/bfc0527d0b204f20d238d8e998f9485b637dd4a3

commit bfc0527d0b204f20d238d8e998f9485b637dd4a3
Author: timloh <timloh@chromium.org>
Date: Tue Nov 08 02:34:20 2016

Disallow CSS custom properties as descriptors

CSS custom properties are properties so should be not allowed where we
only otherwise allow descriptors. This patch makes the parser reject
custom properties in at rules (excluding in keyframe blocks, which take
properties instead of descriptors).

BUG= 641873 

Review-Url: https://codereview.chromium.org/2478623003
Cr-Commit-Position: refs/heads/master@{#430484}

[add] https://crrev.com/bfc0527d0b204f20d238d8e998f9485b637dd4a3/third_party/WebKit/LayoutTests/fast/css/variables/variables-in-at-rules-expected.txt
[add] https://crrev.com/bfc0527d0b204f20d238d8e998f9485b637dd4a3/third_party/WebKit/LayoutTests/fast/css/variables/variables-in-at-rules.html
[modify] https://crrev.com/bfc0527d0b204f20d238d8e998f9485b637dd4a3/third_party/WebKit/Source/core/css/parser/CSSParserImpl.cpp