Cleanup remaining class/struct fwd declarations

tools/gn/ninja_toolchain_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_TOOLCHAIN_WRITER_H_
 #define TOOLS_GN_NINJA_TOOLCHAIN_WRITER_H_
 
 #include <iosfwd>
 #include <set>
 #include <string>
 #include <vector>
 
 #include "base/gtest_prod_util.h"
 #include "base/macros.h"
 #include "tools/gn/ninja_writer.h"
 #include "tools/gn/path_output.h"
 #include "tools/gn/toolchain.h"
 
-class BuildSettings;
 struct EscapeOptions;
 class Settings;
-class Target;
 class Tool;
 
 class NinjaToolchainWriter {
  public:
   // Takes the settings for the toolchain, as well as the list of all targets
   // associated with the toolchain.
   static bool RunAndWriteFile(
       const Settings* settings,
       const Toolchain* toolchain,
       const std::vector<NinjaWriter::TargetRulePair>& rules);
@@ skipping 18 matching lines @@
 
   const Settings* settings_;
   const Toolchain* toolchain_;
   std::ostream& out_;
   PathOutput path_output_;
 
   DISALLOW_COPY_AND_ASSIGN(NinjaToolchainWriter);
 };
 
 #endif  // TOOLS_GN_NINJA_TOOLCHAIN_WRITER_H_