Fix nested config in GN.

build/toolchain/win/midl.gni

 # Copyright 2014 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.
 
 assert(is_win)
 
 import("//build/config/win/visual_studio_version.gni")
 
 # This template defines a rule to invoke the MS IDL compiler.
 #
@@ skipping 66 matching lines @@
       interface_identifier_file,
       proxy_file,
       "{{source}}",
       "/char",
       "signed",
       "/env",
       idl_target_platform,
       "/Oicf",
     ]
 
-    if (defined(invoker.deps)) {
-      deps = invoker.deps
-    }
+    forward_variables_from(invoker, [ "deps" ])
   }
 
   source_set(target_name) {
-    if (defined(invoker.visibility)) {
-      visibility = invoker.visibility
-    }
+    forward_variables_from(invoker, [ "visibility" ])
 
     # We only compile the IID files from the IDL tool rather than all outputs.
     sources = process_file_template(invoker.sources,
                                     [ "$out_dir/$interface_identifier_file" ])
 
     public_deps = [
       ":$action_name",
     ]
 
-    config("midl_warnings") {
-      if (is_clang) {
-        # MIDL generates code like "#endif !_MIDL_USE_GUIDDEF_"
-        cflags = [ "-Wno-extra-tokens" ]
-      }
-    }
-    configs += [ ":midl_warnings" ]
+    configs += [ "//build/config/win:midl_warnings" ]
   }
 }