Fix prototype registration upon SlowToFast migration

Fix prototype registration upon SlowToFast migration

When a prototype object migrates from a slow to a fast map, where the slow map
was registered as a user of its own prototype, then the registration must be
transferred to the new map (just like MigrateToMap does for all other cases).

BUG=chromium:513602
LOG=y
NOTREECHECKS=true

Review URL: https://codereview.chromium.org/1263543004

Cr-Commit-Position: refs/heads/master@{#29898}