Add Persistent::ClearAndLeak.

This will be relevant after Persistent is changed to Dispose itself when
destructed. With Persistent::ClearAndLeak, Blink can take the ownership of the
object pointed by a Persistent and avoid it getting destructed.

BUG=
R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/15023010

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

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14624 ce2b1a6d-e550-0410-aec6-3dcde31c8c00