Previously, fast holey elements accessors would detect element presence
by simply doing a hole check on any slot within the backing store's
capacity. This relied on the (mostly-true but brittle) assumption that
slots beyond the length are always correctly zapped with The Hole.

Review-Url: https://codereview.chromium.org/2297253002
Cr-Commit-Position: refs/heads/master@{#39051}