Allow enable_topchrome_md to be overridden in gn build

build/config/ui.gni

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # This file contains UI-related build flags (see features.gni for Chrome
 # feature-related ones). These should theoretically be moved to the ui
 # directory.
 #
 # However, today we have many "bad" dependencies on some of these flags from,
 # e.g. base, so they need to be global to match the GYP configuration. Also,
@@ skipping 29 matching lines @@
 
   # Whether we should use GTKv3 instead of GTKv2.
   use_gtk3 = false
 
   # Optional system libraries.
   use_xkbcommon = false
 
   # Whether we should use glib, a low level C utility library.
   use_glib = is_linux
 
+  # Indicates if material design elements in the top chrome of the browser are
+  # enabled.
+  enable_topchrome_md = is_chromeos || is_win || is_linux
+
   # Indicates if Wayland display server support is enabled.
   enable_wayland_server = is_chromeos
 }
 
 # Additional dependent variables -----------------------------------------------
 #
 # These variables depend on other variables and can't be set externally.
 
 # Use GPU accelerated cross process image transport by default on linux builds
 # with the Aura window manager.
@@ skipping 21 matching lines @@
   use_cairo = false
   use_pango = false
 }
 
 # Whether to use atk, the Accessibility ToolKit library
 use_atk = is_desktop_linux && use_x11
 
 use_clipboard_aurax11 = is_linux && use_aura && use_x11
 
 enable_hidpi = is_mac || is_win || is_linux
-
-enable_topchrome_md = is_chromeos || is_win || is_linux