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

tools/gn/standard_out.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/standard_out.h"
 
 #include <vector>
 
 #include "base/command_line.h"
 #include "base/logging.h"
@@ skipping 161 matching lines @@
       size_t chars_to_highlight = line.find(':');
       if (chars_to_highlight == std::string::npos)
         chars_to_highlight = line.size();
       OutputString(line.substr(0, chars_to_highlight), DECORATION_YELLOW);
       OutputString(line.substr(chars_to_highlight) + "\n");
       continue;
     }
 
     // Check for a comment.
     TextDecoration dec = DECORATION_NONE;
-    for (size_t char_i = 0; char_i < line.size(); char_i++) {
-      if (line[char_i] == '#') {
+    for (const auto& elem : line) {
+      if (elem == '#') {
         // Got a comment, draw dimmed.
         dec = DECORATION_DIM;
         break;
-      } else if (line[char_i] != ' ') {
+      } else if (elem != ' ') {
         break;
       }
     }
 
     OutputString(line + "\n", dec);
   }
 }