Support private values in GN.

tools/gn/import_manager.cc

 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "tools/gn/import_manager.h"
 
 #include "base/memory/scoped_ptr.h"
 #include "base/stl_util.h"
 #include "tools/gn/parse_tree.h"
 #include "tools/gn/scheduler.h"
@@ skipping 68 matching lines @@
       ImportMap::const_iterator found = imports_.find(file);
       if (found != imports_.end()) {
         delete imported_scope;
         imported_scope = found->second;
       } else {
         imports_[file] = imported_scope;
       }
     }
   }
 
-  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);
 }