Fixed space leak in Interface class.

The exports_ hash map itself should live in the zone, too, not only
its entries.

R=rossberg@chromium.org

Review URL: https://codereview.chromium.org/156643002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19138 ce2b1a6d-e550-0410-aec6-3dcde31c8c00