Web Animations CSS: Record if style recalc is for animation

Based on rolled back patch from issue 86763003

BUG=322573

Review URL: https://codereview.chromium.org/110123005

git-svn-id: svn://svn.chromium.org/blink/trunk@164013 bbb929c8-8fbe-4397-9dbb-9b2b20218538