HasRealIndexedProperty doesn't work on JSGlobalProxy

HasRealIndexedProperty didn't unwrap the JSGlobalProxy and therefore always
returned false.

BUG=257748
R=adamk@chromium.org, rossberg@chromium.org

Review URL: https://codereview.chromium.org/18402007

Patch from Adam Barth <abarth@chromium.org>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15610 ce2b1a6d-e550-0410-aec6-3dcde31c8c00