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

build/config/BUILDCONFIG.gn

Index: build/config/BUILDCONFIG.gn
diff --git a/build/config/BUILDCONFIG.gn b/build/config/BUILDCONFIG.gn
index 942605e38e33022ef00ce2b6066ef71e974dd411..9c3372c5994859007d4acdedf45a1801eb9a1e08 100644
--- a/build/config/BUILDCONFIG.gn
+++ b/build/config/BUILDCONFIG.gn
@@ -130,9 +130,6 @@ declare_args() {
   # separate flags.
   is_official_build = false
 
-  # Debug build. Enabling official builds automatically sets is_debug to false.
-  is_debug = !is_official_build
-
   # Whether we're a traditional desktop unix.
   is_desktop_linux = current_os == "linux"
 
@@ -153,6 +150,11 @@ declare_args() {
 }
 
 declare_args() {
+  # Debug build. Enabling official builds automatically sets is_debug to false.
+  is_debug = !is_official_build
+}
+
+declare_args() {
   # Component build. Setting to true compiles targets declared as "components"
   # as shared libraries loaded dynamically. This speeds up development time.
   # When false, components will be linked statically.