Refactor GN source expansions.

tools/gn/copy_target_generator.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/copy_target_generator.h"
 
 #include "tools/gn/build_settings.h"
 #include "tools/gn/filesystem_utils.h"
 #include "tools/gn/parse_tree.h"
 #include "tools/gn/scope.h"
@@ skipping 17 matching lines @@
     return;
   FillOutputs();
   if (err_->has_error())
     return;
 
   if (target_->sources().empty()) {
     *err_ = Err(function_call_, "Empty sources for copy command.",
         "You have to specify at least one file to copy in the \"sources\".");
     return;
   }
-  if (target_->action_values().outputs().size() != 1) {
+  if (target_->action_values().outputs().list().size() != 1) {
     *err_ = Err(function_call_, "Copy command must have exactly one output.",
         "You must specify exactly one value in the \"outputs\" array for the "
         "destination of the copy\n(see \"gn help copy\"). If there are "
         "multiple sources to copy, use source expansion\n(see \"gn help "
         "source_expansion\").");
     return;
   }
 }