Replace more for loops in GN

tools/gn/substitution_list.cc

 // Copyright 2014 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/substitution_list.h"
 
 #include <string.h>
 
 #include "tools/gn/value.h"
 
@@ skipping 46 matching lines @@
   if (c)
     input_strings.push_back(c);
 
   Err err;
   SubstitutionList result;
   result.Parse(input_strings, NULL, &err);
   return result;
 }
 
 void SubstitutionList::FillRequiredTypes(SubstitutionBits* bits) const {
-  for (size_t i = 0; i < list_.size(); i++)
-    list_[i].FillRequiredTypes(bits);
+  for (const auto& item : list_)
+    item.FillRequiredTypes(bits);
 }