Fix invalid loop condition for Array.lastIndexOf().

BUG=380512
LOG=y
R=jarin@chromium.org

Review URL: https://codereview.chromium.org/313073003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21665 ce2b1a6d-e550-0410-aec6-3dcde31c8c00