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 <avi@chromium.org>
Commit-Queue: Yuta Hijikata <ythjkt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#831498}

diff --git a/ui/aura/BUILD.gn b/ui/aura/BUILD.gn
index 193de1bb..94a749246 100644
--- a/ui/aura/BUILD.gn
+++ b/ui/aura/BUILD.gn

@@ -101,6 +101,7 @@
     "//base",
     "//base:i18n",
     "//base/third_party/dynamic_annotations",
+    "//build:chromeos_buildflags",
     "//cc",
     "//cc/mojo_embedder",
     "//components/discardable_memory/client",
@@ -228,6 +229,7 @@
   deps = [
     "//base",
     "//base/test:test_support",
+    "//build:chromeos_buildflags",
     "//cc:test_support",
     "//components/viz/test:test_support",
     "//skia",
@@ -343,6 +345,7 @@
   deps = [
     ":test_support",
     "//base/test:test_support",
+    "//build:chromeos_buildflags",
     "//cc/mojo_embedder",
     "//components/viz/client",
     "//mojo/core/embedder",