initialize v8::Private with v8::String

R=rossberg@chromium.org

BUG=

Review URL: https://codereview.chromium.org/179983002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19741 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Harmony: implement Math.cbrt in Javascript.

R=jarin@chromium.org

Review URL: https://codereview.chromium.org/183743018

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19742 ce2b1a6d-e550-0410-aec6-3dcde31c8c00