Change GN to disallow reading args defined in the same declare_args() call.

build/config/ui.gni

Index: build/config/ui.gni
diff --git a/build/config/ui.gni b/build/config/ui.gni
index 0f78dd9f0b5418dd1fc7213802bbfb033ad480cd..ab687c47d3740593a1851faa124602f4bc8703e9 100644
--- a/build/config/ui.gni
+++ b/build/config/ui.gni
@@ -33,10 +33,6 @@ declare_args() {
   # of a replacement for GDI or GTK.
   use_aura = is_win || is_linux
 
-  # True means the UI is built using the "views" framework.
-  toolkit_views =
-      (is_mac || is_win || is_chromeos || use_aura) && !is_chromecast
-
   # Whether the entire browser uses toolkit-views on Mac instead of Cocoa.
   mac_views_browser = false
 
@@ -56,6 +52,12 @@ declare_args() {
   enable_vulkan = false
 }
 
+declare_args() {
+  # True means the UI is built using the "views" framework.
+  toolkit_views =
+      (is_mac || is_win || is_chromeos || use_aura) && !is_chromecast
+}
+
 # Additional dependent variables -----------------------------------------------
 #
 # These variables depend on other variables and can't be set externally.