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_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: I34cfe1a8d474a2b146138ce2502c4cb1875c37cb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2494194
Reviewed-by: David Bokan <bokan@chromium.org>
Commit-Queue: Yuta Hijikata <ythjkt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#825694}

diff --git a/cc/BUILD.gn b/cc/BUILD.gn
index 10078c6..3bb5e55 100644
--- a/cc/BUILD.gn
+++ b/cc/BUILD.gn

@@ -417,6 +417,7 @@
   deps = [
     "//base",
     "//base/third_party/dynamic_annotations",
+    "//build:chromeos_buildflags",
     "//components/viz/client",
     "//device/base/synchronization",
     "//gpu",
@@ -800,6 +801,7 @@
     ":cc",
     ":test_support",
     "//base/test:test_support",
+    "//build:chromeos_buildflags",
     "//cc/mojo_embedder",
     "//cc/paint",
     "//components/ukm:test_support",