Check for GN args using the union of all rather than individually.

tools/gn/command_args.cc

 // 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.
 
 #include <map>
 
 #include "base/strings/string_number_conversions.h"
 #include "base/strings/string_util.h"
 #include "tools/gn/commands.h"
 #include "tools/gn/input_file.h"
@@ skipping 105 matching lines @@
     "\n"
     "  If \"arg name\" is specified, only the information for that argument\n"
     "  will be displayed. Otherwise all arguments will be displayed.\n";
 
 int RunArgs(const std::vector<std::string>& args) {
   Setup* setup = new Setup;
   setup->set_check_for_bad_items(false);
   if (!setup->DoSetup() || !setup->Run())
     return 1;
 
-  const Scope::KeyValueMap& build_args =
-      setup->build_settings().build_args().declared_arguments();
+  Scope::KeyValueMap build_args;
+  setup->build_settings().build_args().MergeDeclaredArguments(&build_args);
 
   if (args.size() == 1) {
     // Get help on a specific command.
     Scope::KeyValueMap::const_iterator found_arg = build_args.find(args[0]);
     if (found_arg == build_args.end()) {
       Err(Location(), "Unknown build argument.",
           "You asked for \"" + args[0] + "\" which I didn't find in any "
           "buildfile\nassociated with this build.");
       return 1;
     }
@@ skipping 15 matching lines @@
   for (std::map<base::StringPiece, Value>::iterator i = sorted_args.begin();
        i != sorted_args.end(); ++i) {
     PrintArgHelp(i->first, i->second);
     OutputString("\n");
   }
 
   return 0;
 }
 
 }  // namespace commands