This adds the new runtime flag as a variant and runs it on a subset of
builders corresponding to the "extra" testing set.

Currently failing tests are skipped in the new variant.

After https://crrev.com/c/1433777 this costs only little additional
resources.

Bug: v8:8678
Change-Id: Ibd0e38872814d11252e55a7c6a58d313aa84ebe3
Reviewed-on: https://chromium-review.googlesource.com/c/1433774
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59080}