Also added comments to remind us why we were using the hole here in the first
place (it's used for the case where Object.observe, rather than Array.observe,
has been called on Array that's undergoing truncation).

BUG=356589
LOG=N
R=rossberg@chromium.org

Review URL: https://codereview.chromium.org/213823002

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@20316 ce2b1a6d-e550-0410-aec6-3dcde31c8c00