Currently is_linux is set to true on Chrome OS build,
but it is planned to set to false. This CL is the preparation
to keep the current behavior.

Bug: chromium:1110266
Test: Built locally. Tryjob.
Change-Id: I4124dfb251d68a519fed3e08555d1aa5a694c77c
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2404500Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>