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Background:
Currently lacros-chrome is built with target_os="linux" but we plan to build lacros-chrome with target_os="chromeos" going forward.
To achieve this goal, we have to replace defined(OS_CHROMEOS) with BUILDFLAG(IS_ASH) so not to include code specific to ash-chrome in lacros-chrome.

What this does:
We are replacing existing defined(OS_CHROMEOS) with BUILDFLAG(IS_ASH) with an
exception where is is accompanied with `|| defined(OS_LINUX)` since that
will evaluate to true for both ash-chrome and lacros-chrome.

The buildflag was introduced in http://crrev.com/c/2426071.

For more detail please read the design doc go/lacros-build-config.

Test: printing_unittests
Bug: 1052397
Change-Id: I51902ef0e963c509662576885be5602895685908
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2437079Reviewed-by: Sean Kau <skau@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Yuta Hijikata <ythjkt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#814163}