This prepares to add the stress-marking flag on the infra side.

TBR=sergiyb@chromium.org
NOTRY=true

Bug: v8:6972
Change-Id: Ibee30beadb167d06fd7965dfd3cc05fb523158cb
Reviewed-on: https://chromium-review.googlesource.com/874350Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50697}