Allow GN configs to have sub-configs.

tools/gn/config.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 "tools/gn/config.h"
 
 #include "tools/gn/err.h"
 #include "tools/gn/input_file_manager.h"
 #include "tools/gn/scheduler.h"
 
 Config::Config(const Settings* settings, const Label& label)
-    : Item(settings, label) {
+    : Item(settings, label),
+      resolved_(false) {
 }
 
 Config::~Config() {
 }
 
 Config* Config::AsConfig() {
   return this;
 }
 
 const Config* Config::AsConfig() const {
   return this;
 }
+
+bool Config::OnResolved(Err* err) {
+  DCHECK(!resolved_);
+  resolved_ = true;
+
+  if (!configs_.empty()) {
+    // Nested configs, flatten.

[Dirk Pranke, 2015/09/15 21:31:30]

As noted elsewhere, I don't like the use of "nested" here.

+    composite_values_ = own_values_;
+    for (const auto& pair : configs_)
+      composite_values_.AppendValues(pair.ptr->resolved_values());
+  }
+  return true;
+}