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

tools/gn/ninja_writer.cc

Index: tools/gn/ninja_writer.cc
diff --git a/tools/gn/ninja_writer.cc b/tools/gn/ninja_writer.cc
index 37c982f688e88a42acc899f6807bfed3f7fa1dd8..23b9a72f8f913ff08f1b2ca1b10e430711106795 100644
--- a/tools/gn/ninja_writer.cc
+++ b/tools/gn/ninja_writer.cc
@@ -52,11 +52,11 @@ bool NinjaWriter::WriteToolchains(std::vector<const Settings*>* all_settings,
   CategorizedMap categorized;
 
   std::vector<const BuilderRecord*> all_records = builder_->GetAllRecords();
-  for (size_t i = 0; i < all_records.size(); i++) {
-    if (all_records[i]->type() == BuilderRecord::ITEM_TARGET &&
-        all_records[i]->should_generate()) {
-      categorized[all_records[i]->label().GetToolchainLabel()].push_back(
-          all_records[i]->item()->AsTarget());
+  for (const auto& all_record : all_records) {
+    if (all_record->type() == BuilderRecord::ITEM_TARGET &&
+        all_record->should_generate()) {
+      categorized[all_record->label().GetToolchainLabel()].push_back(
+          all_record->item()->AsTarget());
       }
   }
   if (categorized.empty()) {