Finding a property, but not using an IC, indicates that the variable was
found on the prototype (in DOMWindow). Those properties need to be
ignored while storing global properties via the IC.

Review URL: https://chromiumcodereview.appspot.com/12040039

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@13480 ce2b1a6d-e550-0410-aec6-3dcde31c8c00