Empty Array prototype elements protection needs to alert on length change.

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}