Don't bypass the global proxy as the global object should never escape into JS

BUG=
R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/334233003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21930 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Tiny cleanup, reduce direct usage of the LookupIterator

BUG=
R=ishell@chromium.org

Review URL: https://codereview.chromium.org/346233003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21931 ce2b1a6d-e550-0410-aec6-3dcde31c8c00