When an object shrinks, we can keep the recorded slots until the sweeper
removes them. It is safe because the recorded slots will not be over-
written with untagged values.

BUG=chromium:694255

Change-Id: I2b910c6345a306e00c7a10396876001e663f51ea
Reviewed-on: https://chromium-review.googlesource.com/591650Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46999}