Fix syntax error in validate_web_idl.py

The script is using an undeclared variable "opt_name".

Bug: 1198370
Change-Id: I531aa71d22169d6920e79ca4b425edce20090dea
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2957469
Commit-Queue: Kenichi Ishibashi <bashi@chromium.org>
Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Reviewed-by: Kenichi Ishibashi <bashi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#891935}

third_party/blink/renderer/bindings/scripts/validate_web_idl.py

@@ -32,7 +32,8 @@ def parse_options():
     ]
     for required_option_name in required_option_names:
         if getattr(options, required_option_name) is None:
-            parser.error("--{} is a required option.".format(opt_name))
+            parser.error(
+                "--{} is a required option.".format(required_option_name))
 
     return options, args