This adds a tri-state flag --infra-staging and --no-infra-staging, which
can be used to explicitly opt in or out of the staging test runner.

If not specified, a new architecture whitelist will enable roll-out per
arch for manual (none CI) runs.

We'll start whitelisting archs in follow ups.

Bug: v8:7343
Change-Id: I1228e48969fd379f5c231a2b8fad4afc01da94c0
Reviewed-on: https://chromium-review.googlesource.com/881169
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Sergiy Byelozyorov <sergiyb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50812}