Property backing store size for object literals is
the number of constant and named properties (possibly
over-allocating for the same names).

We do not reserve space in the backing store for __proto__.

We do not reserve space in the backing store for index keys.
Currently, we account for index keys in the runtime when iterating
over the boilerplate properties. Since the boilerplate properties
only include the properties up to the first computed property
name, the property backing store size includes space for index keys
if seen after the first computed property.

R=verwaest@chromium.org

BUG=v8:5625

Review-Url: https://codereview.chromium.org/2650593002
Cr-Commit-Position: refs/heads/master@{#42584}