Introduce FieldIndex to unify and abstract property/field offset

R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/300283002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21720 ce2b1a6d-e550-0410-aec6-3dcde31c8c00