That way, we don't have to implement the fast <-> slow migration logic,
and we don't allocate in-object properties anyways

BUG=chromium:571365
R=verwaest@chromium.org,neis@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/1582773003

Cr-Commit-Position: refs/heads/master@{#33328}