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: Iedb6f15e5aac26bdf6eb17d4017e6fe86f22f951
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2537252
Reviewed-by: Ahmed Fakhry <[email protected]>
Commit-Queue: Yuta Hijikata <[email protected]>
Cr-Commit-Position: refs/heads/master@{#829112}
diff --git a/ui/display/display_features.cc b/ui/display/display_features.cc
index 9d5a08e..a7893af 100644
--- a/ui/display/display_features.cc
+++ b/ui/display/display_features.cc
@@ -5,11 +5,12 @@
 #include "ui/display/display_features.h"
 
 #include "build/build_config.h"
+#include "build/chromeos_buildflags.h"
 
 namespace display {
 namespace features {
 
-#if defined(OS_CHROMEOS)
+#if BUILDFLAG(IS_CHROMEOS_ASH)
 // Enables using HDR transfer function if the monitor says it supports it.
 const base::Feature kUseHDRTransferFunction {
   "UseHDRTransferFunction",