The `shared attribute processing steps for iframe and frame elements` may invoke `iframe load event steps` on frame element

[awesomekling]

https://html.spec.whatwg.org/multipage/iframe-embed-object.html#shared-attribute-processing-steps-for-iframe-and-frame-elements

In step 4.2. we call out to run iframe load event steps given element, which expects an iframe parameter. However, the element at this point may also be a frame.

[domenic]

Thanks! I put a fix for this into #6315 since it seemed like the best way to fix this was by a proper refactoring, and I didn't want to do that twice (once before #6315, and once after). See 81cd346