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This is a mechanical change replacing defined(OS_CHROMEOS) with
BUILDFLAG(IS_CHROMEOS_ASH) to ensure that switching lacros' target_os
from linux to chromeos will not affect the lacros binary.

More detail on http://go/lacros-macros.

Bug: 1052397
Change-Id: Id4b8f4ea0605a897cadbf2140201e2b8efe7acd6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2627653Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Yuta Hijikata <ythjkt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843873}