Allocate room for expected number of properties based on the

Allocate room for expected number of properties based on the
constructor in the JSObject. This removes the need to allocate
a properties array if the object is never assigned any extra
properties.

Review URL: http://codereview.chromium.org/7341

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@501 ce2b1a6d-e550-0410-aec6-3dcde31c8c00