Add GN targets for mojom generator

mojo/public/js/bindings/BUILD.gn

Index: mojo/public/js/bindings/BUILD.gn
diff --git a/net/net.isolate b/mojo/public/js/bindings/BUILD.gn
similarity index 66%
copy from net/net.isolate
copy to mojo/public/js/bindings/BUILD.gn
index f5ca9b501c302711d39d7e465e2fdebbdde0b415..0f305e924ec52d3d1b6923ecc012b55a1ac69eac 100644
--- a/net/net.isolate
+++ b/mojo/public/js/bindings/BUILD.gn
@@ -1,10 +1,10 @@
 # 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.
-{
-  'conditions': [
-  ],
-  'includes': [
-    '../base/base.isolate',
-  ],
+
+source_set("bindings") {
+  sources = [
+    "constants.cc",
+    "constants.h",
+  ]
 }