If the length of the array prototype is changed, be sure to turn off the
guarantee that it's elements are empty.

This case was missed in https://codereview.chromium.org/1092043002
("Protect the emptiness of Array prototype elements with a PropertyCell")

R=jkummerow@chromium.org
BUG=479781
LOG=N

Review URL: https://codereview.chromium.org/1099453007

Cr-Commit-Position: refs/heads/master@{#28033}