Cleanup remaining class/struct fwd declarations

tools/gn/ninja_build_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_BUILD_WRITER_H_
 #define TOOLS_GN_NINJA_BUILD_WRITER_H_
 
 #include <iosfwd>
 #include <map>
 #include <vector>
 
 #include "base/macros.h"
 #include "tools/gn/path_output.h"
 
 class Builder;
 class BuildSettings;
 class Err;
-class Pool;
 class Settings;
 class Target;
 class Toolchain;
 
 // Generates the toplevel "build.ninja" file. This references the individual
 // toolchain files and lists all input .gn files as dependencies of the
 // build itself.
 class NinjaBuildWriter {
  public:
   NinjaBuildWriter(const BuildSettings* settings,
@@ skipping 35 matching lines @@
   std::ostream& dep_out_;
   PathOutput path_output_;
 
   DISALLOW_COPY_AND_ASSIGN(NinjaBuildWriter);
 };
 
 extern const char kNinjaRules_Help[];
 
 #endif  // TOOLS_GN_NINJA_BUILD_WRITER_H_