buffet: Switch to new registration workflow, user-initiated.

BUG=None
TEST=None

Change-Id: I181065640b8f887dcd3b530942b1d7531f49aedd
Reviewed-on: https://chromium-review.googlesource.com/219720
Reviewed-by: Alex Vakulenko <avakulenko@chromium.org>
Commit-Queue: Anton Muhin <antonm@chromium.org>
Tested-by: Anton Muhin <antonm@chromium.org>