Auto merge of #28560 - yvt:fix-nosrc-iframe-load-2, r=jdm

The "process the iframe attributes" steps shouldn't enqueue the iframe load event steps (anymore)

Fixes `HtmlIFrameElement::process_the_iframe_attributes` (which implements [the "process the `iframe` attributes" steps][1]) queueing an element task to execute [the iframe load event steps][2], which causes another `load` event to be fired on the iframe in addition to the one asynchronously generated by [the "create a new nested browsing context" steps][3].

The following timeline shows what happens when a `src`-less `iframe` element is parser-inserted and which step this PR intends to remove. (This does not necessarily match Servo's current behavior exactly - Most notably, `T2`'s steps might run late because the "create a nested browsing context" steps' implementation actually [parses][5] a blank HTML document.)

```diff
  T0 (networking task source)
   | Insert the `iframe` element, causing the following steps to be executed:
   |  | Create a new nested browsing context
   |  |  | Create a new browsing context
   |  |  |  | Completely finish loading the `iframe` element's `Document`
   |  |  |  |  | Queue element task T2.
   |  |
   |  | Process the `iframe` attributes
-  |  |  | Queue element task T1

- T1 (DOM manipulation task source)
-  | Execute the `iframe` load event steps
-  |  | Fire `load` for the `iframe` element

  T2 (DOM manipulation task source)
   | Execute the `iframe` load event steps
   |  | Fire `load` for the `iframe` element
```

This bug likely originates from [a bug][4] in the specification.

[1]: https://html.spec.whatwg.org/multipage/#process-the-iframe-attributes
[2]: https://html.spec.whatwg.org/multipage/#iframe-load-event-steps
[3]: https://html.spec.whatwg.org/multipage/#creating-a-new-nested-browsing-context
[4]: https://github.com/whatwg/html/pull/5797
[5]: https://html.spec.whatwg.org/multipage/#the-end

---
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #24501, #15727, and the `cross-origin-objects-on-new-window.html` part of #26299

---
- [x] There are tests for these changes OR
- [ ] These changes do not require tests because ___

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