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

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

Bug: 1052397
Change-Id: I2018adb0173f1c0ec56e1f0228bb73fa343da83c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2627199Reviewed-by: David Benjamin <davidben@chromium.org>
Commit-Queue: Yuta Hijikata <ythjkt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843872}