Allow enable_topchrome_md to be overridden in gn build

build/config/ui.gni

Index: build/config/ui.gni
diff --git a/build/config/ui.gni b/build/config/ui.gni
index fa4f142ea9f19a00e6597caaf51300a03dc42505..42d59a46c14796cf63509532f77ff3b09a1f40f5 100644
--- a/build/config/ui.gni
+++ b/build/config/ui.gni
@@ -47,6 +47,10 @@ declare_args() {
   # 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
 }
@@ -88,5 +92,3 @@ 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