This reverts commit 3f820ebb.

Revert "[heap, runtime] Avoid redundant clearing of slots in a trimmed array."

This reverts commit ed76f17b.

Reason: canary crashes.

Bug: chromium:752750, chromium:694255
TBR: mlippautz@chromium.org
Change-Id: I57c533e8a3db5f28e7659c5f326fa343627a3ff7
Reviewed-on: https://chromium-review.googlesource.com/607868Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47300}