This pure refactoring is needed for another upcoming CL.

Note that the actual names are still a bit confusing, because this is
still a kind of swiss-army-knife-field. :-/

R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/52633003

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@17472 ce2b1a6d-e550-0410-aec6-3dcde31c8c00