Have StyleResolver / StyleResolverState deal with Document references, not pointers

Have StyleResolver / StyleResolverState deal with Document references, not pointers
since the document can never be NULL. Using references makes it obvious to the
caller.

BUG=281400

Review URL: https://chromiumcodereview.appspot.com/23601011

git-svn-id: svn://svn.chromium.org/blink/trunk@157047 bbb929c8-8fbe-4397-9dbb-9b2b20218538