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

tools/gn/toolchain.cc

Index: tools/gn/toolchain.cc
diff --git a/tools/gn/toolchain.cc b/tools/gn/toolchain.cc
index 341a48cf185c50ff9eaf1ba70ebeaf0cb9f5bd78..dbc1173b466eb8bec11034966d101b0779f59489 100644
--- a/tools/gn/toolchain.cc
+++ b/tools/gn/toolchain.cc
@@ -89,9 +89,9 @@ void Toolchain::SetTool(ToolType type, scoped_ptr<Tool> t) {
 
 void Toolchain::ToolchainSetupComplete() {
   // Collect required bits from all tools.
-  for (size_t i = 0; i < TYPE_NUMTYPES; i++) {
-    if (tools_[i])
-      substitution_bits_.MergeFrom(tools_[i]->substitution_bits());
+  for (const auto& tool : tools_) {
+    if (tool)
+      substitution_bits_.MergeFrom(tool->substitution_bits());
   }
 
   setup_complete_ = true;