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

tools/gn/command_clean.cc

 // Copyright 2015 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 "base/files/file_path.h"
 #include "base/files/file_util.h"
 #include "base/strings/string_split.h"
 #include "base/strings/stringprintf.h"
 #include "tools/gn/commands.h"
 #include "tools/gn/err.h"
@@ skipping 12 matching lines @@
   std::string file_contents;
   if (!base::ReadFileToString(build_ninja_file, &file_contents)) {
     return std::string();
   }
 
   std::vector<std::string> lines;
   base::SplitStringDontTrim(file_contents, '\n', &lines);
 
   std::string result;
   int num_blank_lines = 0;
-  for (size_t i = 0; i < lines.size(); ++i) {
-    result += lines[i];
+  for (const auto& line : lines) {
+    result += line;
     result += "\n";
-    if (lines[i].empty()) {
+    if (line.empty()) {
       ++num_blank_lines;
     }
     if (num_blank_lines == 2)
       break;
   }
 
   return result;
 }
 
 const char kDefaultNinjaFile[] =
@@ skipping 94 matching lines @@
                       static_cast<int>(dummy_content.size())) == -1) {
     Err(Location(), std::string("Failed to write build.ninja.d."))
         .PrintToStdout();
     return 1;
   }
 
   return 0;
 }
 
 }  // namespace commands