The initial fast array may change, invalidating assumptions.

Bug: chromium:798026
Change-Id: Iddcc40867221a2a58aef33b64e7399e0f2784e89
Reviewed-on: https://chromium-review.googlesource.com/850356
Commit-Queue: Michael Stanton <mvstanton@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50363}