[GN] Don't rewrite files with the same contents

tools/gn/ninja_target_writer.cc

 // 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.
 
 #include "tools/gn/ninja_target_writer.h"
 
 #include <sstream>
 
 #include "base/files/file_util.h"
 #include "base/strings/string_util.h"
@@ skipping 56 matching lines @@
   } else if (target->output_type() == Target::GROUP) {
     NinjaGroupTargetWriter writer(target, file);
     writer.Run();
   } else if (target->IsBinary()) {
     NinjaBinaryTargetWriter writer(target, file);
     writer.Run();
   } else {
     CHECK(0) << "Output type of target not handled.";
   }
 
-  std::string contents = file.str();
-  base::WriteFile(ninja_file, contents.c_str(),
-                  static_cast<int>(contents.size()));
+  WriteFileIfChanged(ninja_file, file.str(), nullptr);
 }
 
 void NinjaTargetWriter::WriteEscapedSubstitution(SubstitutionType type) {
   EscapeOptions opts;
   opts.mode = ESCAPE_NINJA;
 
   out_ << kSubstitutionNinjaNames[type] << " = ";
   EscapeStringToStream(out_,
       SubstitutionWriter::GetTargetSubstitution(target_, type),
       opts);
@@ skipping 184 matching lines @@
        << GetNinjaRulePrefixForToolchain(settings_)
        << Toolchain::ToolTypeToName(Toolchain::TYPE_STAMP);
   path_output_.WriteFiles(out_, files);
 
   if (!order_only_deps.empty()) {
     out_ << " ||";
     path_output_.WriteFiles(out_, order_only_deps);
   }
   out_ << std::endl;
 }