This is a reland of 712fa675.

Original change's description:
> [test] Add Liftoff variant
>
> Add a variant for testing the current state of the Liftoff
> implementation.
> This variant will only run on a subset of the bots, just like the
> --future variant.
>
> R=machenbach@chromium.org, hablich@chromium.org
>
> Bug: v8:7088, v8:6600
> Change-Id: If49fad3a8ed579356504b821a787326754f24e78
> Reviewed-on: https://chromium-review.googlesource.com/779420
> Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
> Reviewed-by: Michael Achenbach <machenbach@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#49504}

TBR=machenbach@chromium.org
CC=hablich@chromium.org

Bug: v8:7088, v8:6600
Change-Id: Ieb20020f07c70acaa64bb421763a41aa163a261b
Reviewed-on: https://chromium-review.googlesource.com/781499
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49531}