Don't exchange null and undefined with the global object in function.prototype.{call, apply} for natives.

This makes us compatible with firefox in throwing an exception when
call is invoked on a builtin with null as the this argument.
Review URL: http://codereview.chromium.org/6902104

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@7763 ce2b1a6d-e550-0410-aec6-3dcde31c8c00