https://chromium-review.googlesource.com/561001 was not sufficient,
because we don't use the --gerrit flag in V8 anymore.

Now we explicitly pass an option for private upload.

TBR=hablich@chromium.org,tandrii@chromium.org
NOTRY=true

Bug: v8:6574
Change-Id: Ib8e2883795f26f22ac47e648b37c47f3a0a60a40
Reviewed-on: https://chromium-review.googlesource.com/561378Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46439}