Add support for data deps.

tools/gn/ninja_writer.h

 // 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_WRITER_H_
 #define TOOLS_GN_NINJA_WRITER_H_
 
 #include "base/basictypes.h"
 
 class BuildSettings;
 
 class NinjaWriter {
  public:
+  // On failure will print an error and will return false.
   static bool RunAndWriteFiles(const BuildSettings* build_settings);
 
  private:
   NinjaWriter(const BuildSettings* build_settings);
   ~NinjaWriter();
 
   bool WriteRootBuildfiles();
 
   const BuildSettings* build_settings_;
 
   DISALLOW_COPY_AND_ASSIGN(NinjaWriter);
 };
 
 #endif  // TOOLS_GN_NINJA_WRITER_H_