Allow GN configs to have sub-configs.

tools/gn/config.cc

Index: tools/gn/config.cc
diff --git a/tools/gn/config.cc b/tools/gn/config.cc
index dce951f65b33384fec9403cb65176ef6354662d8..c06632449a4634b0c807f010f7e6ff23088f8279 100644
--- a/tools/gn/config.cc
+++ b/tools/gn/config.cc
@@ -9,7 +9,8 @@
 #include "tools/gn/scheduler.h"
 
 Config::Config(const Settings* settings, const Label& label)
-    : Item(settings, label) {
+    : Item(settings, label),
+      resolved_(false) {
 }
 
 Config::~Config() {
@@ -22,3 +23,29 @@ Config* Config::AsConfig() {
 const Config* Config::AsConfig() const {
   return this;
 }
+
+bool Config::OnResolved(Err* err) {
+  DCHECK(!resolved_);
+  resolved_ = true;
+
+  if (!configs_.empty()) {
+    // Subconfigs, flatten.
+    //
+    // Implementation note for the future: Flattening these here means we
+    // lose the ability to de-dupe subconfigs. If a subconfig is listed as
+    // a separate config or a subconfig that also applies to the target, the
+    // subconfig's flags will be duplicated.
+    //
+    // If we want to be able to de-dupe these, here's one idea. As a config is
+    // resolved, inline any sub-sub configs so the configs_ vector is a flat
+    // list, much the same way that libs and lib_dirs are pushed through
+    // targets. Do the same for Target.configs_ when a target is resolved. This
+    // will naturally de-dupe and also prevents recursive config walking to
+    // compute every possible flag, although it will expand the configs list on
+    // a target nontrivially (depending on build configuration).
+    composite_values_ = own_values_;
+    for (const auto& pair : configs_)
+      composite_values_.AppendValues(pair.ptr->resolved_values());
+  }
+  return true;
+}