The current "dense" format is not expressive enough to distinguish between
an element that is not defined and one that has the value "undefined",
but in this CL the existing behaviour of Blink is used for such cases.
Format changes to fix these issues could be made later on.

Not included in this CL is compatibility with version 0 arrays.
Those will be implemented in a separate CL.

BUG=chromium:148757

Review-Url: https://codereview.chromium.org/2259633002
Cr-Commit-Position: refs/heads/master@{#38729}