Tighten the ElementResolveContext API

This removes document() since there was only one real caller of it
and the code is more clear with just element()->document() instead.

It also turns out that isDocumentElement() was never called and that
rootElementStyle() can be const.

Review URL: https://codereview.chromium.org/27284002

git-svn-id: svn://svn.chromium.org/blink/trunk@159649 bbb929c8-8fbe-4397-9dbb-9b2b20218538