Currently is_linux GN variable is set to true on building Chrome OS
but it is planned to be set false. This CL is the preparation to
keep the compatibility.

Bug: chromium:1110266
Test: Built locally.
Change-Id: Ibb9a57269f5a147e372fd33a473d9514379e1c68
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2405847Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69870}