LaCrOS: Replace defined(OS_CHROMEOS) with BUILDFLAG(IS_CHROMEOS_ASH)

The change is mostly mechanical replacing defined(OS_CHROMEOS) with
BUILDFLAG(IS_CHROMEOS_ASH) and GN variable is_chromeos with
is_chromeos_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: I0b849ad0a6bacdb757163329831f2f0ef1852a83
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2560530
Reviewed-by: Avi Drissman <[email protected]>
Commit-Queue: Yuta Hijikata <[email protected]>
Cr-Commit-Position: refs/heads/master@{#831498}
91 files changed