1) they always own descriptors,
2) the number of own descriptors is equal to the number of descriptors in the descriptor array.

This allows an initial map of a subclass to share descriptor array with initial map of the parent class (if it already contains properties).

BUG=chromium:551430
LOG=N

Review URL: https://codereview.chromium.org/1411933005

Cr-Commit-Position: refs/heads/master@{#31892}