The --predictable flag is often used to reproduce issues, and having it
imply --single-threaded can change decisions like which compiler(s) to
use. This is because --single-threaded is meant to be set by embedders
(hence we do our best to support single-threaded execution), whereas
--predictable is a testing-only flag which should not change semantics
too much. The fact that --predictable executes everything in a single
thread is already implied by the PredictablePlatform.

R=ahaas@chromium.org, machenbach@chromium.org
CC=​jkummerow@chromium.org

Change-Id: Ic174dd59dfdbd6aa1a410f983db05db26c944cd5
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2919828
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74822}