Implement document load cancellation

[jdm]

This has two parts to it:

• make pressing escape in the default embedding cancel the ongoing load of the foreground, top-most document
• cause all ongoing subresource loads for the document to be cancelled when the document is cancelled

The easiest way to tie the loads together might be to move the cancellation sender into the LoadType enum.

Blocked on #19274.

[gterzian]

If no one is doing this, I would be interested...

[gterzian]

I propose making this about implementing the following:

1. https://html.spec.whatwg.org/multipage/window-object.html#dom-window-stop
2. https://html.spec.whatwg.org/multipage/browsing-the-web.html#abort-a-document

and also adding a hook at the embedding level via the escape key, to perform a window.stop() on the "foreground, top-most document", or perhaps 'the active tab' is another way to put it.

By the way, I think such a hook is covered by this part of the spec: "User agents may allow users to explicitly invoke the abort a document algorithm for a Document. If the user does so, then, if that Document is an active document, the user agent should queue a task to fire an event named abort at that Document's Window object before invoking the abort algorithm."

Currently we don't have the concept of an "foreground, top-most document/active tab", so maybe I'll end implementing it in some simple form as part of this too @paulrouget

[paulrouget]

Works for me.

[gterzian]

Also worth noting:

""abort" Event Isn't Fired When Aborting Document Load" whatwg/html#3525

"add test for aborting a document load" web-platform-tests/wpt#9933

"Add tests for window.stop" web-platform-tests/wpt#11154

[gterzian]

Also worth noting:
When to "fail the connection" of an EventSource? whatwg/html#3789