Dealing with this case requires a wire format change. It is possible that an
element can be absent even in an array where the dense format was chosen
(because the array initially had no holes), if the elements are modified while
they are being serialized. In this case, a new tag for the "hole" is emitted.

The logic to treat undefined in dense arrays as an absent property is restricted
to versions of the wire format that this tag did not exist.

BUG=chromium:686159,chromium:665820

Review-Url: https://codereview.chromium.org/2660093002
Cr-Commit-Position: refs/heads/master@{#42784}