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

tools/gn/operators.cc

Index: tools/gn/operators.cc
diff --git a/tools/gn/operators.cc b/tools/gn/operators.cc
index 17e31a728ab398556f09f89e9b82eee883fc9d13..b78e9350df2a9b1db36441df5e39b1e7cafcbb09 100644
--- a/tools/gn/operators.cc
+++ b/tools/gn/operators.cc
@@ -98,10 +98,10 @@ void RemoveMatchesFromList(const BinaryOpNode* op_node,
     }
 
     case Value::LIST:  // Filter out each individual thing.
-      for (size_t i = 0; i < to_remove.list_value().size(); i++) {
+      for (const auto& elem : to_remove.list_value()) {
         // TODO(brettw) if the nested item is a list, we may want to search
         // for the literal list rather than remote the items in it.
-        RemoveMatchesFromList(op_node, list, to_remove.list_value()[i], err);
+        RemoveMatchesFromList(op_node, list, elem, err);
         if (err->has_error())
           return;
       }