Move field index into property details, freeing up the value slot of fields.

BUG=
R=jkummerow@chromium.org

Review URL: https://chromiumcodereview.appspot.com/15941016

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14909 ce2b1a6d-e550-0410-aec6-3dcde31c8c00