Allow GN configs to have sub-configs.

tools/gn/config_values.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_values.h"
 
+namespace {
+
+template<typename T>
+void VectorAppend(std::vector<T>* append_to,
+                  const std::vector<T>& append_this) {
+  if (append_this.empty())
+    return;
+  append_to->insert(append_to->end(),append_this.begin(), append_this.end());
+}
+
+}  // namespace
+
 ConfigValues::ConfigValues() {
 }
 
 ConfigValues::~ConfigValues() {
 }
+
+void ConfigValues::AppendValues(const ConfigValues& append) {
+  VectorAppend(&cflags_,       append.cflags_);
+  VectorAppend(&cflags_c_,     append.cflags_c_);
+  VectorAppend(&cflags_cc_,    append.cflags_cc_);
+  VectorAppend(&cflags_objc_,  append.cflags_objc_);
+  VectorAppend(&cflags_objcc_, append.cflags_objcc_);
+  VectorAppend(&defines_,      append.defines_);
+  VectorAppend(&include_dirs_, append.include_dirs_);
+  VectorAppend(&ldflags_,      append.ldflags_);
+  VectorAppend(&lib_dirs_,     append.lib_dirs_);
+  VectorAppend(&libs_,         append.libs_);
+
+  // Only append precompiled header if there isn't one. It might be nice to
+  // throw an error if there are conflicting precompiled headers, but that
+  // requires piping through some context of the actual configs involved, and
+  // conflicts here should be very unusual. Instead, use the first value.
+  if (!append.precompiled_header_.empty() && !precompiled_header_.empty())
+    precompiled_header_ = append.precompiled_header_;
+  if (!append.precompiled_source_.is_null() && !precompiled_source_.is_null())
+    precompiled_source_ = append.precompiled_source_;
+}