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This is a mechanical change 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.

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

Bug: 1052397
Change-Id: Ia047bcc761acac4773b8d62930771e89cb6d8c09
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2628870
Commit-Queue: Yuta Hijikata <ythjkt@chromium.org>
Reviewed-by: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843948}