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

tools/gn/standard_out.cc

Index: tools/gn/standard_out.cc
diff --git a/tools/gn/standard_out.cc b/tools/gn/standard_out.cc
index b318fe477a9cb50914c374121dfc4b5329ccda92..69d443be368a22f37b8f1ca4eee2c6fe642dbaeb 100644
--- a/tools/gn/standard_out.cc
+++ b/tools/gn/standard_out.cc
@@ -179,12 +179,12 @@ void PrintLongHelp(const std::string& text) {
 
     // 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;
       }
     }