Ensure we set a parent style on StyleResolverState when applying animations

This is a regression from https://codereview.chromium.org/635203003. That CL 
introduced a path where no parentStyle is set on the StyleResolverState if
animations are applied.

BUG=424108

Review URL: https://codereview.chromium.org/660563006

git-svn-id: svn://svn.chromium.org/blink/trunk@184683 bbb929c8-8fbe-4397-9dbb-9b2b20218538