Flip the --harmony_object_own_property_descriptors flag to enabled by default.

Intent-to-Ship at https://groups.google.com/forum/#!topic/v8-users/xhTKRuY0_V8

BUG=v8:4725
R=adamk@chromium.org, littledan@chromium.org

Review-Url: https://codereview.chromium.org/2191023002
Cr-Commit-Position: refs/heads/master@{#38181}