since the old style weakness is slated for removal, we might as well reuse the name to limit confusion.  additionally I simplified the callback type to a enum to either get internal field values or not

this should be a non-breaking change with the exception of PhantomPersistentValueMap, which is unused.

R=jochen@chromium.org, erikcorry@chromium.org

BUG=

Review URL: https://codereview.chromium.org/989153003

Cr-Commit-Position: refs/heads/master@{#27084}