Bug 1805414 - Remove nsMenuFrame and nsMenuParent. r=smaug,Jamie,desktop-theme-reviewers,settings-reviewers,dao

Move most the event handling stuff to the DOM. I've left nsMenuBarFrame
for now, but I will be removing that in the future.

The basic set up is:

  * nsMenuParent becomes XULMenuParentElement (menubar or popup, manages
    the current active menu item)

  * nsMenuFrame -> XULButtonElements that return true for IsMenu().
    Can't use XULMenuElement because of <button type=menu>, which
    behaves like a, well, menu.

This makes the a11y events for menus (DOMMenuItem{Active,Inactive}) make
sense (before that we were firing duplicate Inactive events etc, and the
event order was rather suspicious).

Differential Revision: https://phabricator.services.mozilla.com/D164210

layout/base/PresShell.cpp

@@ -150,7 +150,6 @@
 // For style data reconstruction
 #include "nsStyleChangeList.h"
 #include "nsCSSFrameConstructor.h"
-#include "nsMenuFrame.h"
 #include "nsTreeBodyFrame.h"
 #include "XULTreeElement.h"
 #include "nsMenuPopupFrame.h"
@@ -8851,12 +8850,11 @@ bool PresShell::EventHandler::AdjustContextMenuKeyEvent(
     WidgetMouseEvent* aMouseEvent) {
   // if a menu is open, open the context menu relative to the active item on the
   // menu.
-  nsXULPopupManager* pm = nsXULPopupManager::GetInstance();
-  if (pm) {
+  if (nsXULPopupManager* pm = nsXULPopupManager::GetInstance()) {
     nsIFrame* popupFrame = pm->GetTopPopup(ePopupTypeMenu);
     if (popupFrame) {
-      nsIFrame* itemFrame =
-          (static_cast<nsMenuPopupFrame*>(popupFrame))->GetCurrentMenuItem();
+      nsIFrame* itemFrame = (static_cast<nsMenuPopupFrame*>(popupFrame))
+                                ->GetCurrentMenuItemFrame();
       if (!itemFrame) itemFrame = popupFrame;
 
       nsCOMPtr<nsIWidget> widget = popupFrame->GetNearestWidget();