X87: Introduce FieldIndex to unify and abstract property/field offset

port r21720.

Original commit message:
   Introduce FieldIndex to unify and abstract property/field offset.

R=danno@chromium.org

Review URL: https://codereview.chromium.org/326563002

Patch from Chunyang Dai <chunyang.dai@intel.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21724 ce2b1a6d-e550-0410-aec6-3dcde31c8c00