Add GN support for midl.

third_party/iaccessible2/BUILD.gn

+# 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/toolchain/win/midl.gni")
+
+midl("iaccessible2") {
+  sources = [
+    "ia2_api_all.idl",
+  ]
+}
+
+# Generate a proxy DLL from the generated code.
+shared_library("IAccessible2Proxy") {
+  sources = [
+    "IAccessible2Proxy.def",
+    "$target_gen_dir/ia2_api_all.dlldata.c",
+    "$target_gen_dir/ia2_api_all_p.c",
+  ]
+
+  defines = [ "REGISTER_PROXY_DLL" ]
+
+  libs = [ "rpcrt4.lib" ]
+
+  deps = [ ":iaccessible2" ]
+}