BUG=89490
TEST=manual: "Infinity = 42;" doesn't change the value of "Infinity"

Review URL: http://codereview.chromium.org/7457019

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@8691 ce2b1a6d-e550-0410-aec6-3dcde31c8c00