Make Object.freeze fast

This patch both speeds up the freeze operation itself, but also
allows properties to remain in fast mode. Objects with non-empty
elements backing stores still end up with slow elements.

BUG=v8:1858, 115960
R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/14888005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14758 ce2b1a6d-e550-0410-aec6-3dcde31c8c00