Unify LookupIterator::GetRoot and Objects::GetRootMap

BUG=none
R=verwaest@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/388193002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22369 ce2b1a6d-e550-0410-aec6-3dcde31c8c00