Bug 1750072 - Allow to prevent password revealing/unrevealing. r=mtigley

The idea is that chrome code would be able to call preventDefault(),
then run primary password authentication when needed, then setRevealPassword
again.

Differential Revision: https://phabricator.services.mozilla.com/D173491

dom/html/HTMLInputElement.cpp

@@ -6876,6 +6876,20 @@ void HTMLInputElement::SetRevealPassword(bool aValue) {
   if (NS_WARN_IF(mType != FormControlType::InputPassword)) {
     return;
   }
+  if (aValue == State().HasState(ElementState::REVEALED)) {
+    return;
+  }
+  RefPtr doc = OwnerDoc();
+  // We allow chrome code to prevent this. This is important for about:logins,
+  // which may need to run some OS-dependent authentication code before
+  // revealing the saved passwords.
+  bool defaultAction = true;
+  nsContentUtils::DispatchEventOnlyToChrome(
+      doc, ToSupports(this), u"MozWillToggleReveal"_ns, CanBubble::eYes,
+      Cancelable::eYes, &defaultAction);
+  if (NS_WARN_IF(!defaultAction)) {
+    return;
+  }
   if (aValue) {
     AddStates(ElementState::REVEALED);
   } else {