Load the global proxy from the context of the target function.

BUG=
R=dcarney@chromium.org

Review URL: https://codereview.chromium.org/111613003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18458 ce2b1a6d-e550-0410-aec6-3dcde31c8c00