Support private values in GN.

tools/gn/import_manager.cc

Index: tools/gn/import_manager.cc
diff --git a/tools/gn/import_manager.cc b/tools/gn/import_manager.cc
index 4bb4b63eee731b3c2cccc834499c7c2e53a58c59..842577cfb7ac87ab5f9fbbbaa02a9f01ed20324a 100644
--- a/tools/gn/import_manager.cc
+++ b/tools/gn/import_manager.cc
@@ -86,6 +86,9 @@ bool ImportManager::DoImport(const SourceFile& file,
     }
   }
 
-  return imported_scope->NonRecursiveMergeTo(scope, false, node_for_err,
+  Scope::MergeOptions options;
+  options.skip_private_vars = true;
+  options.mark_used = true;  // Don't require all imported values be used.
+  return imported_scope->NonRecursiveMergeTo(scope, options, node_for_err,
                                              "import", err);
 }