Cleanup remaining class/struct fwd declarations

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 <map>
 #include <set>
 #include <string>
 #include <vector>
 
 #include "base/macros.h"
 
 class Builder;
 class BuildSettings;
 class Err;
-class Pool;
-class Settings;
 class Target;
 class Toolchain;
 
 class NinjaWriter {
  public:
   // Combines a target and the computed build rule for it.
   using TargetRulePair = std::pair<const Target*, std::string>;
 
   // Associates the build rules with each toolchain.
   using PerToolchainRules =
@@ skipping 14 matching lines @@
 
   bool WriteToolchains(const PerToolchainRules& per_toolchain_rules,
                        Err* err);
 
   const Builder& builder_;
 
   DISALLOW_COPY_AND_ASSIGN(NinjaWriter);
 };
 
 #endif  // TOOLS_GN_NINJA_WRITER_H_