fixed contexts slots. Take this into account when using
the new, fast context creation path to avoid allocating
too many slots (wasteful).
Review URL: http://codereview.chromium.org/501148

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@3505 ce2b1a6d-e550-0410-aec6-3dcde31c8c00