Free up 11 bits in fast-mode PropertyDetails by removing the enumeration-index.

The descriptors are nowadays ordered in order of addition, so that info was
duplicated.

Review URL: https://chromiumcodereview.appspot.com/14622005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14571 ce2b1a6d-e550-0410-aec6-3dcde31c8c00