Update Array Iterator to use numeric indexes

At the last face-to-face meeting it was decided that we should use
numeric indexes for the Array Iterator values.

https://github.com/rwldrn/tc39-notes/blob/master/es6/2013-07/july-24.md#514-keys-entries-return-numbers-for-array-index-properties

BUG=v8:2818
R=dslomov@chromium.org

Review URL: https://codereview.chromium.org/21180008

Patch from Erik Arvidsson <arv@chromium.org>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16072 ce2b1a6d-e550-0410-aec6-3dcde31c8c00