Enable -Wextra-tokens on win clang.

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 83 matching lines @@
       visibility = 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" ]
   }
 }