This is convenient when using identity hashes in data structures that
want to reserve 0 as a sentinel value, such as WebKit's WTF::HashMap.

Review URL: http://codereview.chromium.org/100147

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@1821 ce2b1a6d-e550-0410-aec6-3dcde31c8c00