Check @apply syntax in custom properties declarations at parse time

This patch moves syntax checking for @apply references inside custom
property declarations from style computation to parse time. This makes
us reject any custom property declarations which has an invalid @apply
rule, such as "--foo: @apply bar".

BUG=586974

Review URL: https://codereview.chromium.org/1752773003

Cr-Commit-Position: refs/heads/master@{#379747}
4 files changed