Implementing document.setRootScroller API for main thread scrolling.

third_party/WebKit/Source/core/input/EventHandler.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2006 Alexey Proskuryakov (ap@webkit.org)
  * Copyright (C) 2012 Digia Plc. and/or its subsidiary(-ies)
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
@@ skipping 2487 matching lines @@
 }
 
 bool EventHandler::isRootScroller(const Node& node) const
 {
     // The root scroller is the one Element on the page designated to perform
     // "viewport actions" like top controls movement and overscroll glow.
 
     if (!node.isElementNode() || node.document().ownerElement())
         return false;
 
-    Element* scrollingElement = node.document().scrollingElement();
-    return scrollingElement
-        ? toElement(&node) == node.document().scrollingElement()
-        : toElement(&node) == node.document().documentElement();
+    return node.document().rootScroller() == toElement(&node);
 }
 
 WebInputEventResult EventHandler::handleGestureScrollUpdate(const PlatformGestureEvent& gestureEvent)
 {
     ASSERT(gestureEvent.type() == PlatformEvent::GestureScrollUpdate);
 
     // Negate the deltas since the gesture event stores finger movement and
     // scrolling occurs in the direction opposite the finger's movement
     // direction. e.g. Finger moving up has negative event delta but causes the
     // page to scroll down causing positive scroll delta.
@@ skipping 1626 matching lines @@
 
 FrameHost* EventHandler::frameHost()
 {
     if (!m_frame->page())
         return nullptr;
 
     return &m_frame->page()->frameHost();
 }
 
 } // namespace blink