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

build/config/BUILDCONFIG.gn

 # Copyright (c) 2013 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.
 
 # =============================================================================
 # WHAT IS THIS FILE?
 # =============================================================================
 #
 # This is the master GN build configuration. This file is loaded after the
 # build args (args.gn) for the build directory and after the toplevel ".gn"
@@ skipping 112 matching lines @@
 #   even if the value is overridden, which is wasteful. See first bullet.
 
 declare_args() {
   # Set to enable the official build level of optimization. This has nothing
   # to do with branding, but enables an additional level of optimization above
   # release (!is_debug). This might be better expressed as a tri-state
   # (debug, release, official) but for historical reasons there are two
   # 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"
 
   # Set to true when compiling with the Clang compiler. Typically this is used
   # to configure warnings.
   is_clang = current_os == "mac" || current_os == "ios" ||
              current_os == "linux" || current_os == "chromeos"
 
   # Allows the path to a custom target toolchain to be injected as a single
   # argument, and set as the default toolchain.
   custom_toolchain = ""
 
   # This should not normally be set as a build argument.  It's here so that
   # every toolchain can pass through the "global" value via toolchain_args().
   host_toolchain = ""
 
   # DON'T ADD MORE FLAGS HERE. Read the comment above.
 }
 
 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.
   #
   # For more information see
   # https://chromium.googlesource.com/chromium/src/+/master/docs/component_build.md
   is_component_build = is_debug && current_os != "ios"
 }
 
 assert(!(is_debug && is_official_build), "Can't do official debug builds")
@@ skipping 477 matching lines @@
 set_defaults("component") {
   if (is_component_build) {
     configs = default_shared_library_configs
     if (is_android) {
       configs -= [ "//build/config/android:hide_native_jni_exports" ]
     }
   } else {
     configs = default_compiler_configs
   }
 }