script: improve spec compliance of the iframe load event steps (#40100)

Prevent the running of the iframe load event steps for the initial
about:blank document. Part of
https://github.com/servo/servo/issues/31973

Testing:
/html/browsers/browsing-the-web/navigating-across-documents/initial-empty-document/iframe-nosrc.html
Fixes: It is expected that the intermittent issue of 34819 will go away,
but I will leave it open so we can close it when we realize it isn't
being mentioned by PR's anymore.

https://github.com/servo/servo/issues/34819

---------

Signed-off-by: gterzian <2792687+gterzian@users.noreply.github.com>

components/script/dom/html/htmlformelement.rs

@@ -80,6 +80,7 @@ use crate::dom::node::{
 use crate::dom::nodelist::{NodeList, RadioListMode};
 use crate::dom::radionodelist::RadioNodeList;
 use crate::dom::submitevent::SubmitEvent;
+use crate::dom::types::HTMLIFrameElement;
 use crate::dom::virtualmethods::VirtualMethods;
 use crate::dom::window::Window;
 use crate::links::{LinkRelations, get_element_target};
@@ -1061,6 +1062,17 @@ impl HTMLFormElement {
         load_data.referrer = referrer;
         load_data.referrer_policy = referrer_policy;
 
+        // Note the pending form navigation if this is an iframe;
+        // necessary for deciding whether to run the iframe load event steps.
+        if let Some(window_proxy) = target.undiscarded_window_proxy() {
+            if let Some(frame) = window_proxy
+                .frame_element()
+                .and_then(|e| e.downcast::<HTMLIFrameElement>())
+            {
+                frame.note_pending_navigation()
+            }
+        }
+
         // 4. Queue an element task on the DOM manipulation task source
         // given the form element and the following steps:
         let form = Trusted::new(self);