Fix second half of issue 1151, the first change (r6765) only fixed FunctionGetPrototype, not FunctionSetPrototype.

Review URL: http://codereview.chromium.org/6548008

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@6893 ce2b1a6d-e550-0410-aec6-3dcde31c8c00