Bug 1754905 - Match Pivot API in nsIAccessiblePivot. r=Jamie

This patch does several things:
1. Remove virtual cursor doc accessible member.
2. Remove state-keeping nsAccessiblePivot implementation and replace it
   with something that is much more similar to the native Pivot
   interface.
3. Move nsIAccessiblePivot implementation to xpcom and rename it
   xpcAccessiblePivot.
4. Delete mochitests and introduce browser tests.
5. Make new xpcom implementation work on remote accessibles as well.

Differential Revision: https://phabricator.services.mozilla.com/D181813

accessible/base/NotificationController.cpp

@@ -7,6 +7,7 @@
 
 #include "DocAccessible-inl.h"
 #include "DocAccessibleChild.h"
+#include "LocalAccessible-inl.h"
 #include "nsEventShell.h"
 #include "TextLeafAccessible.h"
 #include "TextUpdater.h"