When a (prototype) map registers as a user of its own prototype, it now remembers the index in that prototype's registry where it is listed.
This remembered index is used on un-registration to find the right slot to clear without walking the entire registry.
Compaction of the registry must update all entries' remembered indices.

BUG=chromium:517778,chromium:517406
LOG=n
R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/1276353004

Cr-Commit-Position: refs/heads/master@{#30079}