Define the "extract an origin" operation

whatwg/html#11846 added an `Origin` interface, which relies on an "extract an origin" operation to support the creation of new `Origin` objects via `Origin.from(...)`. This PR defines that operation for `URL` objects.

Addresses a remaining portion of whatwg/html#11534.

Author: mikewest

url.bs

@@ -3494,6 +3494,11 @@ null-or-<a>scalar value string</a> <var>base</var> (default null), and then runs
 </ol>
 </div>
 
+<div algorithm="URL/extract an origin">
+<p>Objects implementing the {{URL}} interface's <a for="platform object">extract an origin</a> steps are
+to return <a>this</a>'s <a for=URL>URL</a>'s <a for=url>origin</a>. [[!HTML]]
+</div>
+
 <hr>
 
 <div algorithm>