The serializer won't ever write a more complex object. Not validating this
allows other things to be used as keys, and converted to string when the
property set actually occurs. It turns out this gives an opportunity to trigger
OOM by giving an object a key which is a very large sparse array (whose string
representation is very large).

This case is now rejected by the deserializer.

BUG=chromium:686511

Review-Url: https://codereview.chromium.org/2697023002
Cr-Commit-Position: refs/heads/master@{#43249}