[cleanup] Remove Node::FocusDelegate

Follow up of:
https://chromium-review.googlesource.com/c/chromium/src/+/2363325

There should be no behaviour change.

Change-Id: I435a75bcc51bcf17acd17fc9079bbb563cd587aa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2365913
Commit-Queue: Ian Kilpatrick <ikilpatrick@chromium.org>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#799969}

third_party/blink/renderer/core/dom/node.cc

@@ -1505,10 +1505,6 @@ bool Node::InActiveDocument() const {
   return isConnected() && GetDocument().IsActive();
 }
 
-const Node* Node::FocusDelegate() const {
-  return this;
-}
-
 bool Node::ShouldHaveFocusAppearance() const {
   DCHECK(IsFocused());
   return true;

third_party/blink/renderer/core/dom/node.h

@@ -599,7 +599,6 @@ class CORE_EXPORT Node : public EventTarget {
   void SetHasFocusWithin(bool flag);
   virtual void SetDragged(bool flag);
 
-  virtual const Node* FocusDelegate() const;
   // This is called only when the node is focused.
   virtual bool ShouldHaveFocusAppearance() const;
 

third_party/blink/renderer/core/html/forms/slider_thumb_element.cc

@@ -91,10 +91,6 @@ bool SliderThumbElement::MatchesReadWritePseudoClass() const {
   return HostInput() && HostInput()->MatchesReadWritePseudoClass();
 }
 
-const Node* SliderThumbElement::FocusDelegate() const {
-  return HostInput();
-}
-
 void SliderThumbElement::DragFrom(const LayoutPoint& point) {
   StartDragging();
   SetPositionFromPoint(point);

third_party/blink/renderer/core/html/forms/slider_thumb_element.h

@@ -68,7 +68,6 @@ class SliderThumbElement final : public HTMLDivElement {
   bool IsDisabledFormControl() const override;
   bool MatchesReadOnlyPseudoClass() const override;
   bool MatchesReadWritePseudoClass() const override;
-  const Node* FocusDelegate() const override;
   void StartDragging();
 
   bool