Don't pass the hole to SetElement when creating Array.observe change records

Don't pass the hole to SetElement when creating Array.observe change records

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

Committed: https://code.google.com/p/v8/source/detail?r=20316

[rossberg, 2014-03-27 08:34:41]

LGTM

[adamk, 2014-03-27 18:29:11]

Committed patchset #1 manually as r20316 (presubmit successful).