Add template parameter to ReturnValue::Set.

E.g., v8-i18n wants to set the return value with a different type of a Persistent.

BUG=NONE
R=dcarney@chromium.org, svenpanne@chromium.org

Review URL: https://codereview.chromium.org/16102002

Patch from Marja Hölttä <marja@chromium.org>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14826 ce2b1a6d-e550-0410-aec6-3dcde31c8c00