Reorder Build Flag Assert After Includes

Some variables depend on //build/config/chromeos/ui_mode.gni.

Change-Id: I86db3177d316d3a73ef37ea4f50727c4a7a37e2f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2782940
Commit-Queue: Robert Liao <robliao@chromium.org>
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Auto-Submit: Robert Liao <robliao@chromium.org>
Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#865878}

ui/gtk/BUILD.gn

@@ -2,9 +2,6 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-assert(is_linux || is_chromeos_lacros || is_chromeos,
-       "This file should only be referenced on Linux")
-
 import("//build/buildflag_header.gni")
 import("//build/config/chromeos/ui_mode.gni")
 import("//build/config/features.gni")
@@ -14,6 +11,9 @@ import("//build/config/ui.gni")
 import("//printing/buildflags/buildflags.gni")
 import("//tools/generate_stubs/rules.gni")
 
+assert(is_linux || is_chromeos_lacros || is_chromeos,
+       "This file should only be referenced on Linux")
+
 buildflag_header("gtk_buildflags") {
   header = "gtk_buildflags.h"