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This change is a mechanical change replacing is_chromeos with
is_chromeos_ash.

This is in preparation to switching lacros from target_os="linux" to
target_os="chromeos".

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

Bug: 1052397
Change-Id: I1324b2733d438d2fe62eaa54363b364ad0257862
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2628631Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Peng Huang <penghuang@chromium.org>
Commit-Queue: Yuta Hijikata <ythjkt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#845496}