Implement PersistentValueMap, a map that stores UniquePersistent values.
This is preparatory work to get rid of UnsafePersistent in blink.

Related blink changes are here: https://codereview.chromium.org/180363004/

This patch is largely based on https://codereview.chromium.org/175503003/, with some methods added to support the blink change mentioned above.

BUG=
R=dcarney@chromium.org, svenpanne@chromium.org

Review URL: https://codereview.chromium.org/189463019

Patch from Daniel Vogelheim <vogelheim@chromium.org>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19833 ce2b1a6d-e550-0410-aec6-3dcde31c8c00