tools/gn: Convert for loops to use the new range-based loops in C++11.

tools/gn/ninja_toolchain_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_toolchain_writer.h"
 
 #include <fstream>
 
 #include "base/files/file_util.h"
 #include "base/strings/stringize_macros.h"
@@ skipping 110 matching lines @@
                                             const EscapeOptions& options) {
   if (pattern.empty())
     return;
   out_ << kIndent << name << " = ";
   SubstitutionWriter::WriteWithNinjaVariables(pattern, options, out_);
   out_ << std::endl;
 }
 
 void NinjaToolchainWriter::WriteSubninjas() {
   // Write subninja commands for each generated target.
-  for (size_t i = 0; i < targets_.size(); i++) {
-    OutputFile ninja_file(targets_[i]->settings()->build_settings(),
-                          GetNinjaFileForTarget(targets_[i]));
+  for (const auto& target: targets_) {
+    OutputFile ninja_file(target->settings()->build_settings(),
+                          GetNinjaFileForTarget(target));
     out_ << "subninja ";
     path_output_.WriteFile(out_, ninja_file);
     out_ << std::endl;
   }
   out_ << std::endl;
 }