Remove StyleResolver::documentSettings()

This is the only reason that Document.h needs to be included, so
removing this will allow that include to be removed (later).

Review URL: https://codereview.chromium.org/342373004

git-svn-id: svn://svn.chromium.org/blink/trunk@176643 bbb929c8-8fbe-4397-9dbb-9b2b20218538