Compute registered properties after resolving high priority properties

This patch moves computation of registered properties after that of
high priority properties. This makes em units work correctly when
the font-size is set on a given element.

Currently properties which are inherited will be re-computed since we
don't have a distinction between inherited properties and those which
we actually set. This does not affect correctness as computation is
idempotent.

As we resolve var() references as token streams, we don't issues with
cycles (e.g. --font-size: 10em; font-size: var(--font-size)). There's
an outstanding issue on the Houdini GitHub to work out what we should
actually do here.

https://github.com/w3c/css-houdini-drafts/issues/315

BUG=641877

Review-Url: https://codereview.chromium.org/2406203004
Cr-Commit-Position: refs/heads/master@{#424697}
6 files changed