Cleanup remaining class/struct fwd declarations

tools/gn/ninja_copy_target_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_COPY_TARGET_WRITER_H_
 #define TOOLS_GN_NINJA_COPY_TARGET_WRITER_H_
 
 #include "base/macros.h"
 #include "tools/gn/ninja_target_writer.h"
 
-class Tool;
-
 // Writes a .ninja file for a copy target type.
 class NinjaCopyTargetWriter : public NinjaTargetWriter {
  public:
   NinjaCopyTargetWriter(const Target* target, std::ostream& out);
   ~NinjaCopyTargetWriter() override;
 
   void Run() override;
 
  private:
   // Writes the rules top copy the file(s), putting the computed output file
   // name(s) into the given vector.
   void WriteCopyRules(std::vector<OutputFile>* output_files);
 
   DISALLOW_COPY_AND_ASSIGN(NinjaCopyTargetWriter);
 };
 
 #endif  // TOOLS_GN_NINJA_COPY_TARGET_WRITER_H_