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

It also renames chromeos_is_browser_only with is_chromeos_lacros which
was a gn var introduced to be used in place of chromeos_is_browser_only.

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

Bug: 1052397
Change-Id: I7e3e5685549c2b8c655a788a9fe827805988b11d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2628711
Commit-Queue: Yuta Hijikata <ythjkt@chromium.org>
Reviewed-by: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843958}