Bug 1166995 - P2. Call onFormSubmit when a form or a password field is removed from the DOM tree r=sfoster,tgiles,smaug

This patch adds two events - `DOMFormRemoved` and `DOMInputPasswordRemoved`.
These events are notified when a form/password field is removed from
the DOM tree. When the LoginManagerChild receives the event, it calls
_onFormSubmit as the form is submitted.

This patch also adds a WeakFieldSet formlessModifiedPasswordFields to
record all the form-less password fields that users have interacted
with to track whether a formless password is removed

Depends on D106024

Differential Revision: https://phabricator.services.mozilla.com/D106025

dom/html/HTMLFormElement.cpp

@@ -438,6 +438,8 @@ static void CollectOrphans(nsINode* aRemovalRoot,
 }
 
 void HTMLFormElement::UnbindFromTree(bool aNullParent) {
+  MaybeFireFormRemoved();
+
   // Note, this is explicitly using uncomposed doc, since we count
   // only forms in document.
   RefPtr<Document> oldDocument = GetUncomposedDoc();
@@ -2416,4 +2418,26 @@ bool HTMLFormElement::IsSubmitting() const {
   return loading;
 }
 
+void HTMLFormElement::MaybeFireFormRemoved() {
+  // We want this event to be fired only when the form is removed from the DOM
+  // tree, not when it is released (ex, tab is closed). So don't fire an event
+  // when the form doesn't have a docshell.
+  Document* doc = GetComposedDoc();
+  nsIDocShell* container = doc ? doc->GetDocShell() : nullptr;
+  if (!container) {
+    return;
+  }
+
+  // Right now, only the password manager listens to the event and only listen
+  // to it under certain circumstances. So don't fire this event unless
+  // necessary.
+  if (!doc->ShouldNotifyFormOrPasswordRemoved()) {
+    return;
+  }
+
+  RefPtr<AsyncEventDispatcher> asyncDispatcher = new AsyncEventDispatcher(
+      this, u"DOMFormRemoved"_ns, CanBubble::eNo, ChromeOnlyDispatch::eYes);
+  asyncDispatcher->RunDOMEventWhenSafe();
+}
+
 }  // namespace mozilla::dom