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The change is mostly mechanical replacing defined(OS_CHROMEOS) with
BUILDFLAG(IS_CHROMEOS_ASH) and GN variable is_chromeos with is_ash
with some special cases (For those cases please refer to
http://go/lacros-macros).

The patch is made in preparation to switching lacros build from
target_os=linux to target_os=chromeos. This will prevent lacros from
changing behaviour after the switch.

Bug: 1052397
Change-Id: I5ec5132b286ab2f67e971abca49c72f9117870bd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2494401
Commit-Queue: Yuta Hijikata <ythjkt@chromium.org>
Reviewed-by: Michael Ershov <miersh@google.com>
Reviewed-by: Ryan Sleevi <rsleevi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826695}