Remove unused byte from Map::instance_sizes field.

Remove unused byte from Map::instance_sizes field.

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.

R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/1226203011

Cr-Commit-Position: refs/heads/master@{#29590}