We do not want to reserve space in the backing store for index keys.
Count index keys during creation of the BoilerplateDescription, and
substract them for the backing store size.

Correctly count index keys after encountering a property with
a computed name during object literal creation.

R=verwaest@chromium.org

BUG=v8:5625

Review-Url: https://codereview.chromium.org/2651523002
Cr-Commit-Position: refs/heads/master@{#42598}