Mojo JS bindings: add support to generate "dual-mode" code.

mojo/public/tools/bindings/mojom_bindings_generator.py

Index: mojo/public/tools/bindings/mojom_bindings_generator.py
diff --git a/mojo/public/tools/bindings/mojom_bindings_generator.py b/mojo/public/tools/bindings/mojom_bindings_generator.py
index c8ffc6e168cbc9780dd6cbb81f2a82227fe556c6..a7f62337695dcdc07d93d482efed2ec8171a452f 100755
--- a/mojo/public/tools/bindings/mojom_bindings_generator.py
+++ b/mojo/public/tools/bindings/mojom_bindings_generator.py
@@ -204,7 +204,7 @@ class MojomProcessor(object):
             variant=args.variant, bytecode_path=args.bytecode_path,
             for_blink=args.for_blink,
             use_once_callback=args.use_once_callback,
-            use_new_js_bindings=args.use_new_js_bindings,
+            js_bindings_mode=args.js_bindings_mode,
             export_attribute=args.export_attribute,
             export_header=args.export_header,
             generate_non_variant_code=args.generate_non_variant_code)
@@ -336,9 +336,12 @@ def main():
       "--use_once_callback", action="store_true",
       help="Use base::OnceCallback instead of base::RepeatingCallback.")
   generate_parser.add_argument(
-      "--use_new_js_bindings", action="store_true",
-      help="Use the new module loading approach and the core API exposed by "
-      "Web IDL. This option only affects the JavaScript bindings.")
+      "--js_bindings_mode", choices=["new", "both", "old"], default="new",
+      help="This option only affects the JavaScript bindings. The value could "
+      "be: \"new\" - generate only the new-style JS bindings, which use the "
+      "new module loading approach and the core api exposed by Web IDL; "
+      "\"both\" - generate both the old- and new-style bindings; \"old\" - "
+      "generate only the old-style bindings.")
   generate_parser.add_argument(
       "--export_attribute", default="",
       help="Optional attribute to specify on class declaration to export it "