Bug 1943226 - Make nsFrameSelection treat limiters are elements r=jjaschke,dom-core

`nsFrameSelection::GetLimiter()` is not `nullptr` only when it's an instance for
an independent selection of a text control.  In the case, it's set to the editor
root anonymous `<div>` of the text control.  Despite the name, this is already
optimized only for this purpose in `nsFrameSelection::NodeIsInLimiters()`.
Thus, we don't have any problems to make this clearer for the other developers
with renaming some parameter names.

`nsFrameSelection::GetAncestorLimiter()` is also always an `Element`.  So,
we can change this to `Element` too.

Differential Revision: https://phabricator.services.mozilla.com/D235254

editor/libeditor/HTMLEditor.cpp

@@ -1063,7 +1063,7 @@ nsresult HTMLEditor::CollapseSelectionToEndOfLastLeafNodeOfDocument() const {
 }
 
 void HTMLEditor::InitializeSelectionAncestorLimit(
-    nsIContent& aAncestorLimit) const {
+    Element& aAncestorLimit) const {
   MOZ_ASSERT(IsEditActionDataAvailable());
 
   // Hack for initializing selection.
@@ -7450,7 +7450,7 @@ void HTMLEditor::NotifyEditingHostMaybeChanged() {
   }
 
   // Compute current editing host.
-  nsIContent* editingHost = ComputeEditingHost();
+  Element* const editingHost = ComputeEditingHost();
   if (NS_WARN_IF(!editingHost)) {
     return;
   }