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

tools/gn/ninja_copy_target_writer.cc

Index: tools/gn/ninja_copy_target_writer.cc
diff --git a/tools/gn/ninja_copy_target_writer.cc b/tools/gn/ninja_copy_target_writer.cc
index 9cd1b72771d5f29a3f375c9133c0043e48106dac..b0313bec3f279dc55bc02a7cbaf1f629d2fa57b4 100644
--- a/tools/gn/ninja_copy_target_writer.cc
+++ b/tools/gn/ninja_copy_target_writer.cc
@@ -100,9 +100,7 @@ void NinjaCopyTargetWriter::WriteCopyRules(
   // where a command might need to make sure something else runs before it runs
   // to avoid conflicts. Such cases should be avoided where possible, but
   // sometimes that's not possible.
-  for (size_t i = 0; i < target_->sources().size(); i++) {
-    const SourceFile& input_file = target_->sources()[i];
-
+  for (const auto& input_file : target_->sources()) {
     OutputFile output_file =
         SubstitutionWriter::ApplyPatternToSourceAsOutputFile(
             target_->settings(), output_subst, input_file);