LibWeb: Don't return an opaque origin for file:// URLs

The protocol of the origin is used for checking if the a file://
iframe is allowed to be loaded (a document with a file:// origin
can load other files in iframes).

This used to be the case, but was changed in
6e71e400e696a92bbc2a69e6a59efddc29b926ce, which broke file:// iframes.

Userland/Libraries/LibWeb/HTML/BrowsingContext.cpp

@@ -52,7 +52,8 @@ static HTML::Origin url_origin(AK::URL const& url)
 
     if (url.scheme() == "file"sv) {
         // Unfortunate as it is, this is left as an exercise to the reader. When in doubt, return a new opaque origin.
-        return HTML::Origin {};
+        // Note: We must return an origin with the `file://' protocol for `file://' iframes to work from `file://' pages.
+        return HTML::Origin(url.protocol(), String(), 0);
     }
 
     return HTML::Origin {};