src: clean up WHATWG WG parser

* reduce indentation
* refactor URL inlined methods
* prefer templates over macros
* do not export ARG_* flags in url binding

PR-URL: #12251
Reviewed-By: Anna Henningsen <[email protected]>
Reviewed-By: Daijiro Wachi <[email protected]>
Reviewed-By: James M Snell <[email protected]>

Author: TimothyGu