GN: Generate error if multiple rules generate same file

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 <vector>
 
@@ skipping 17 matching lines @@
 
  private:
   NinjaBuildWriter(const BuildSettings* settings,
                    const std::vector<const Settings*>& all_settings,
                    const Toolchain* default_toolchain,
                    const std::vector<const Target*>& default_toolchain_targets,
                    std::ostream& out,
                    std::ostream& dep_out);
   ~NinjaBuildWriter();
 
-  void Run();
+  bool Run();
 
   void WriteNinjaRules();
   void WriteLinkPool();
   void WriteSubninjas();
-  void WritePhonyAndAllRules();
+  bool WritePhonyAndAllRules();
 
   void WritePhonyRule(const Target* target, const OutputFile& target_file,
                       const std::string& phony_name);
 
   const BuildSettings* build_settings_;
   std::vector<const Settings*> all_settings_;
   const Toolchain* default_toolchain_;
   std::vector<const Target*> default_toolchain_targets_;
   std::ostream& out_;
   std::ostream& dep_out_;
   PathOutput path_output_;
 
   DISALLOW_COPY_AND_ASSIGN(NinjaBuildWriter);
 };
 
 #endif  // TOOLS_GN_NINJA_BUILD_GENERATOR_H_