We should not expose a raw pointer to typed array's backing store.

R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/25221002

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@16999 ce2b1a6d-e550-0410-aec6-3dcde31c8c00