`webRequest.onBeforeRequest` gives `main_frame` instead of `sub_frame`

For subframes like documents, the webRequest.onBeforeRequest fires with details.type set to `main_frame` instead of `sub_frame`. This matches the optional filter from the listener. As in, if your filter only allows `sub_frame`, no requests will show up.

See: https://jeurissen.co/webext-demos/webrequest-type-sub-frame

WebKit bug report:
https://bugs.webkit.org/show_bug.cgi?id=304828

Start with the linked reproduction demo and read WebKit bug 304828 to understand the reported behavior and current discussion. Done means subframe requests are reported as sub_frame and a listener filtered to sub_frame receives them.

Written by the indexing model from the issue text.