This moves the responsibility of putting a new context into a persistent
handle to the embedder. Also it removes one API function where the copy
constructor for persistent handles is needed.

R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/13799003

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@14203 ce2b1a6d-e550-0410-aec6-3dcde31c8c00