The problem is other array may have holes, for example
when fixed array comes from JSArray (in case of named interceptor).

If that would prove to be a performance problem, we could
pass an additional argument into UnionOfKeys to hold actual length.

Review URL: http://codereview.chromium.org/3595013

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@5591 ce2b1a6d-e550-0410-aec6-3dcde31c8c00