In the case of a simple fast-mode receiver without fancy properties, we
can just walk over the descriptor array to find all its initial property
names. As long as the map stays the same, we can also use that
descriptor array to figure out how to handle the properties.

This speeds up
https://github.com/kpdecker/six-speed/tree/master/tests/object-assign by
~2x.

BUG=

Review URL: https://codereview.chromium.org/1688953004

Cr-Commit-Position: refs/heads/master@{#33895}