Bug 1735745 - Make SelectionChangeEventDispatcher dispatch select event and selectchange event only when there may be corresponding event listeners r=smaug

`select` event on text controls now dispatched immediately before
`selectionchange`.  However, it needs to create `AsyncEventDispatcher` for
each.  This cost may not be expensive, but they are called really a lot even
if there is no corresponding event listener.

Therefore, this patch makes `nsPIDOMWindow` and `EventListenerManager` have
`MayHave*EventListeners` flag separately for each, and makes
`SelectionChangeEventDispatcher` does not try to do create
`AsyncEventDispatcher` when there is no corresponding event listener.

Differential Revision: https://phabricator.services.mozilla.com/D131750

dom/html/TextControlState.cpp

@@ -859,9 +859,13 @@ void TextInputListener::OnSelectionChange(Selection& aSelection,
   // Mozilla: If we have non-empty selection we will fire a new event for each
   //          keypress (or mouseup) if the selection changed. Mozilla will also
   //          create the event each time select all is called, even if
-  //          everything was previously selected, becase technically select all
+  //          everything was previously selected, because technically select all
   //          will first collapse and then extend. Mozilla will never create an
   //          event if the selection collapses to nothing.
+  // FYI: If you want to skip dispatching eFormSelect event and if there are no
+  //      event listeners, you can refer
+  //      nsPIDOMWindow::HasFormSelectEventListeners(), but be careful about
+  //      some C++ event handlers, e.g., HTMLTextAreaElement::PostHandleEvent().
   bool collapsed = aSelection.IsCollapsed();
   if (!collapsed && (aReason & (nsISelectionListener::MOUSEUP_REASON |
                                 nsISelectionListener::KEYPRESS_REASON |