Avoid some unnecessary fast-properties map creations.

(1) When we have just normalized and re-fastified a map, we don't need to copy it again to set the is_prototype bit.
(2) When defining accessors causes a non-prototype object to go slow, don't force re-fastification.

BUG=v8:3267
LOG=n
R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/706243002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@25221 ce2b1a6d-e550-0410-aec6-3dcde31c8c00