Introduce NonJSProxyHolder returning Handle<JSObject> and return Handle<JSReceiver> for GetHolder

BUG=
R=rossberg@chromium.org

Review URL: https://codereview.chromium.org/415953005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22594 ce2b1a6d-e550-0410-aec6-3dcde31c8c00