Add initial prototype for the GN meta-buildsystem.

tools/gn/ninja_toolchain_writer.h

Index: tools/gn/ninja_toolchain_writer.h
diff --git a/tools/gn/ninja_toolchain_writer.h b/tools/gn/ninja_toolchain_writer.h
new file mode 100644
index 0000000000000000000000000000000000000000..71759efa80b3592cf5dac593d5ef5aed19c31c06
--- /dev/null
+++ b/tools/gn/ninja_toolchain_writer.h
@@ -0,0 +1,46 @@
+// 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.
+
+#ifndef TOOLS_GN_NINJA_TOOLCHAIN_WRITER_H_
+#define TOOLS_GN_NINJA_TOOLCHAIN_WRITER_H_
+
+#include <iosfwd>
+#include <vector>
+
+#include "tools/gn/ninja_helper.h"
+#include "tools/gn/path_output.h"
+
+class BuildSettings;
+class Settings;
+class Target;
+
+class NinjaToolchainWriter {
+ public:
+  // Takes the settings for the toolchain, as well as the list of all targets
+  // assicoated with the toolchain.
+  static bool RunAndWriteFile(const Settings* settings,
+                              const std::vector<const Target*>& targets);
+
+ private:
+  NinjaToolchainWriter(const Settings* settings,
+                       const std::vector<const Target*>& targets,
+                       std::ostream& out);
+  ~NinjaToolchainWriter();
+
+  void Run();
+
+  void WriteRules();
+  void WriteSubninjas();
+
+  const Settings* settings_;
+  std::vector<const Target*> targets_;
+  std::ostream& out_;
+  PathOutput path_output_;
+
+  NinjaHelper helper_;
+
+  DISALLOW_COPY_AND_ASSIGN(NinjaToolchainWriter);
+};
+
+#endif  // TOOLS_GN_NINJA_TOOLCHAIN_WRITER_H_