Use float for scroll offset.

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

 /*
  * Copyright (C) 2011 Apple Inc. All rights reserved.
  *
  * 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.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 432 matching lines @@
   if (scrollLayer) {
     bool isForVisualViewport =
         scrollableArea == &m_page->frameHost().visualViewport();
     scrollLayer->setScrollableArea(scrollableArea, isForVisualViewport);
   }
 
   WebLayer* webLayer = toWebLayer(scrollableArea->layerForScrolling());
   WebLayer* containerLayer = toWebLayer(scrollableArea->layerForContainer());
   if (webLayer) {
     webLayer->setScrollClipLayer(containerLayer);
-    DoublePoint scrollPosition(scrollableArea->scrollPositionDouble() +
-                               toDoubleSize(scrollableArea->scrollOrigin()));
+    DoublePoint scrollPosition(FloatPoint(scrollableArea->scrollOrigin()) +
+                               scrollableArea->scrollOffset());
     webLayer->setScrollPositionDouble(scrollPosition);
 
     webLayer->setBounds(scrollableArea->contentsSize());
     bool canScrollX = scrollableArea->userInputScrollable(HorizontalScrollbar);
     bool canScrollY = scrollableArea->userInputScrollable(VerticalScrollbar);
     webLayer->setUserScrollable(canScrollX, canScrollY);
   }
   if (WebScrollbarLayer* scrollbarLayer =
           getWebScrollbarLayer(scrollableArea, HorizontalScrollbar)) {
     GraphicsLayer* horizontalScrollbarLayer =
@@ skipping 764 matching lines @@
   if (frameIsScrollable != m_wasFrameScrollable)
     return true;
 
   if (WebLayer* scrollLayer =
           frameView ? toWebLayer(frameView->layerForScrolling()) : nullptr)
     return WebSize(frameView->contentsSize()) != scrollLayer->bounds();
   return false;
 }
 
 }  // namespace blink