lacros: Migrate chromeos/chromeos_buildflags.h to build/chromeos_buildflags.h

This CL unifies those two buildflags file for Chrome OS.

BUG=1114198
TEST=Build locally. Ran tryjob.

Change-Id: Id7343da06ac6b17cb20cb20babdd8ab06ac40028
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2377525
Commit-Queue: Hidehiko Abe <hidehiko@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#804308}

diff --git a/build/BUILD.gn b/build/BUILD.gn
index 83216c1..16f23564 100644
--- a/build/BUILD.gn
+++ b/build/BUILD.gn

@@ -5,6 +5,7 @@
 import("//build/buildflag_header.gni")
 import("//build/config/chrome_build.gni")
 import("//build/config/chromecast_build.gni")
+import("//build/config/chromeos/args.gni")
 import("//build/config/chromeos/ui_mode.gni")
 
 source_set("buildflag_header_h") {
@@ -36,5 +37,8 @@
 buildflag_header("chromeos_buildflags") {
   header = "chromeos_buildflags.h"
 
-  flags = [ "IS_LACROS=$chromeos_is_browser_only" ]
+  flags = [
+    "IS_CHROMEOS_DEVICE=$is_chromeos_device",
+    "IS_LACROS=$chromeos_is_browser_only",
+  ]
 }