Only supports constructing new objects and returning size.
Followup patch will need to add ability to retrieve and
set contents in order to support structured clone.

Also removes a bunch of outdated "experimental" markers from v8.h.

BUG=v8:3340
LOG=y

Review URL: https://codereview.chromium.org/1157453002

Cr-Commit-Position: refs/heads/master@{#28637}