GN foreach should mark the list identifier as used.

tools/gn/function_foreach.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/err.h"
 #include "tools/gn/functions.h"
 #include "tools/gn/parse_tree.h"
 #include "tools/gn/scope.h"
 
 namespace functions {
@@ skipping 52 matching lines @@
     *err = Err(args_vector[0], "Expected an identifier for the loop var.");
     return Value();
   }
   base::StringPiece loop_var(identifier->value().value());
 
   // Extract the list, avoid a copy if it's an identifier (common case).
   Value value_storage_for_exec;  // Backing for list_value when we need to exec.
   const Value* list_value = NULL;
   const IdentifierNode* list_identifier = args_vector[1]->AsIdentifier();
   if (list_identifier) {
-    list_value = scope->GetValue(list_identifier->value().value());
+    list_value = scope->GetValue(list_identifier->value().value(), true);
     if (!list_value) {
       *err = Err(args_vector[1], "Undefined identifier.");
       return Value();
     }
   } else {
     // Not an identifier, evaluate the node to get the result.
     Scope list_exec_scope(scope);
     value_storage_for_exec = args_vector[1]->Execute(scope, err);
     if (err->has_error())
       return Value();
@@ skipping 31 matching lines @@
     scope->SetValue(loop_var, old_loop_value, old_loop_value.origin());
   } else {
     // Loop variable was undefined before loop, delete it.
     scope->RemoveIdentifier(loop_var);
   }
 
   return Value();
 }
 
 }  // namespace functions