Support private values in GN.

tools/gn/loader.cc

Index: tools/gn/loader.cc
diff --git a/tools/gn/loader.cc b/tools/gn/loader.cc
index 4cca2ce71b2805f5eeb9004a55cba0a0630c2853..32879ea63d3a3e513d9f0ce8054482da26c3deed 100644
--- a/tools/gn/loader.cc
+++ b/tools/gn/loader.cc
@@ -245,6 +245,9 @@ void LoaderImpl::BackgroundLoadFile(const Settings* settings,
   if (err.has_error())
     g_scheduler->FailWithError(err);
 
+  if (!our_scope.CheckForUnusedVars(&err))
+    g_scheduler->FailWithError(err);
+
   // Pass all of the items that were defined off to the builder.
   for (size_t i = 0; i < collected_items.size(); i++)
     settings->build_settings()->ItemDefined(collected_items[i]->Pass());
@@ -285,6 +288,11 @@ void LoaderImpl::BackgroundLoadBuildConfig(
   Err err;
   root_block->ExecuteBlockInScope(base_config, &err);
 
+  // Clear all private variables left in the scope. We want the root build
+  // config to be like a .gni file in that variables beginning with an
+  // underscore aren't exported.
+  base_config->RemovePrivateIdentifiers();
+
   trace.Done();
 
   if (err.has_error())