Fix nested config in GN.

build/toolchain/win/midl.gni

Index: build/toolchain/win/midl.gni
diff --git a/build/toolchain/win/midl.gni b/build/toolchain/win/midl.gni
index 3e7fbecca5392a51730a24617d55bd1fc52dad07..96acd66ea0939f8bf416036e406e2dd0f52f323b 100644
--- a/build/toolchain/win/midl.gni
+++ b/build/toolchain/win/midl.gni
@@ -84,15 +84,11 @@ template("midl") {
       "/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,
@@ -102,12 +98,6 @@ template("midl") {
       ":$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" ]
   }
 }