Disable Supervised User creation on ChromeOS

This adds a disabled-by-default feature SupervisedUserCreation, and
checks it on ChromeOS. A follow-up CL should check the same feature on
desktop.

Bug: 736040
Change-Id: I8e04a5220b8a0f50b4af06eccfa36c084daa815f
Reviewed-on: https://chromium-review.googlesource.com/544929
Reviewed-by: Bernhard Bauer <bauerb@chromium.org>
Reviewed-by: Markus Heintz <markusheintz@chromium.org>
Commit-Queue: Bernhard Bauer <bauerb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#482566}
3 files changed