This reverts commit d1e1f1be.

Reason for revert: Main bug was fixed.

Original change's description:
> [infra] Switch back to Xenial on a slow variant
>
> This is to mitigate flaky timeouts due to memory problems on some
> Bionic bots.
>
> Bug: v8:11818
> Change-Id: I4758f0f167b94d81f43e183a5599a39d8545b4e1
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2922245
> Commit-Queue: Michael Achenbach <machenbach@chromium.org>
> Commit-Queue: Maya Lekova <mslekova@chromium.org>
> Auto-Submit: Michael Achenbach <machenbach@chromium.org>
> Reviewed-by: Maya Lekova <mslekova@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#74824}

Bug: v8:11818
Change-Id: I9719cbcd074de44cd5fb981c40f46bf4e10adee0
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2940895
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Maya Lekova <mslekova@chromium.org>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Auto-Submit: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Maya Lekova <mslekova@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74961}