Animations: Fix rem units not calculated in pseudo element keyframes

Before a pseudo-element is created, rem units are not correctly
calculated since the element is null and the document root is found via
this element. Fix simply involves passing the document root to
ElementResolveContext when the element is null.

BUG=392081

Review URL: https://codereview.chromium.org/395073002

git-svn-id: svn://svn.chromium.org/blink/trunk@178815 bbb929c8-8fbe-4397-9dbb-9b2b20218538