Port 2027335f

Original commit message:
Note that there are currently no objects that require a pre-allocated
properties backing store, all such slots are in-object properties from
the begining. Hence {unused + pre_allocated - inobject == 0} holds.

BUG=

Review URL: https://codereview.chromium.org/1226363003

Cr-Commit-Position: refs/heads/master@{#29610}