Allow setting custom properties from @apply rules

This patch adds support for setting custom properties in @apply rules.
The model implemented here is that custom properties get a first value,
which can be used in @apply rules (not including those inside custom
property values), and then may get a second (final) value after @apply
rules are applied.

BUG=586974

Review URL: https://codereview.chromium.org/1724183007

Cr-Commit-Position: refs/heads/master@{#379507}
8 files changed