New GC related APIs: Implicit references.

When using these APIs, the embedder doesn't need to copy Persistent handles around.

BUG=NONE

Review URL: https://codereview.chromium.org/14175005

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

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14283 ce2b1a6d-e550-0410-aec6-3dcde31c8c00