PersistentValueMap is used to hold per-world wrappers in the blink. Currently,
when we trace wrappers, we visit wrappers in all worlds via this PersistentValueMap. This cl introduces convenient (and faster) way of registering these external references.

BUG=468240
LOG=no

Review URL: https://codereview.chromium.org/1883043003

Cr-Commit-Position: refs/heads/master@{#35523}