Ensure prototypes always stay fast by turning them fast again after an operation that turned them slow

BUG=
R=ishell@chromium.org

Review URL: https://codereview.chromium.org/439243005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22827 ce2b1a6d-e550-0410-aec6-3dcde31c8c00