The slots outside the trimmed array will be cleared by the sweeper and
will not be overwritten with an untagged value.

BUG=chromium:694255

Change-Id: I3e814b9934ca95a09e883e237687434e6bb58c80
Reviewed-on: https://chromium-review.googlesource.com/591651Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46998}