This umblocks switching this builder to Xenial. The corresponding gcc
version is not yet supported by goma.

NOTRY=true

Bug: chromium:933093
Change-Id: Ib00d191c229bb598b209dcaf447321b874571e62
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1505577Reviewed-by: Tamer Tas <tmrts@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60087}