Add ASSERT to SmallMapList::Add to ensure no deprecated maps are ever added.

BUG=
R=danno@chromium.org

Review URL: https://codereview.chromium.org/15179004

Patch from Toon Verwaest <verwaest@chromium.org>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@14675 ce2b1a6d-e550-0410-aec6-3dcde31c8c00