Stop retrieving scrolling element id from the CompositorElementId.

third_party/WebKit/Source/core/page/scrolling/ScrollState.cpp

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "core/page/scrolling/ScrollState.h"
 
 #include <memory>
 #include "core/dom/DOMNodeIds.h"
 #include "core/dom/Element.h"
 #include "core/dom/ExceptionCode.h"
@@ skipping 74 matching lines @@
   data_->delta_y -= y;
 
   if (x)
     data_->caused_scroll_x = true;
   if (y)
     data_->caused_scroll_y = true;
   if (x || y)
     data_->delta_consumed_for_scroll_sequence = true;
 }
 
-Element* ScrollState::CurrentNativeScrollingElement() const {
-  uint64_t dom_node_id =
-      IdFromCompositorElementId(data_->current_native_scrolling_element());
-  if (dom_node_id == kInvalidDOMNodeId)
+Element* ScrollState::CurrentNativeScrollingElement() {
+  if (data_->current_native_scrolling_element() == CompositorElementId()) {
+    element_.Clear();

[pdr., 2017/05/17 00:24:19]

Instead of clearing element_ in the getter, can you clear it in
SetCurrentNativeScrollingElement?

     return nullptr;
-  return ElementForId(dom_node_id);
+  }
+  return element_;
 }
 
 void ScrollState::SetCurrentNativeScrollingElement(Element* element) {
+  element_ = element;
   data_->set_current_native_scrolling_element(CompositorElementIdFromDOMNodeId(
       DOMNodeIds::IdForNode(element), CompositorElementIdNamespace::kScroll));
 }
 
-void ScrollState::SetCurrentNativeScrollingElementById(int element_id) {
-  data_->set_current_native_scrolling_element(CompositorElementIdFromDOMNodeId(
-      element_id, CompositorElementIdNamespace::kScroll));
-}
-
 }  // namespace blink