Makes declared_arguments per toolchain

tools/gn/args_unittest.cc

+// Copyright 2015 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 "tools/gn/args.h"
+
+#include "testing/gtest/include/gtest/gtest.h"
+#include "tools/gn/scheduler.h"
+#include "tools/gn/test_with_scope.h"
+
+TEST(ArgsTest, DeclareArgsMultipleTimes) {
+  TestWithScope setup1, setup2;
+  Args args;
+  Scope::KeyValueMap key_value_map1;
+  Err err;
+  LiteralNode assignment1;
+
+  setup1.scope()->SetValue("a", Value(nullptr, true), &assignment1);
+  setup1.scope()->GetCurrentScopeValues(&key_value_map1);
+  EXPECT_TRUE(args.DeclareArgs(key_value_map1, setup1.scope(), &err));
+
+  LiteralNode assignment2;
+  setup2.scope()->SetValue("a", Value(nullptr, true), &assignment2);
+  Scope::KeyValueMap key_value_map2;
+  setup2.scope()->GetCurrentScopeValues(&key_value_map2);
+  EXPECT_TRUE(args.DeclareArgs(key_value_map2, setup2.scope(), &err));
+}