Allow JSArray fast moving elements even if the array's proto isn't Array.prototype in original state
Otherwise array builtins don't work on internal arrays.

BUG=v8:3681
LOG=n
R=machenbach@chromium.org, mvstanton@chromium.org

Review URL: https://codereview.chromium.org/706703005

Cr-Commit-Position: refs/heads/master@{#25190}
git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@25190 ce2b1a6d-e550-0410-aec6-3dcde31c8c00