This derives two global variables to ease adding new variants.

Bug: chromium:788104
Change-Id: Ibe933dc0999f6dfa0ae8bf2afa0a5de8e44aefca
Reviewed-on: https://chromium-review.googlesource.com/799790
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Sergiy Byelozyorov <sergiyb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49741}