Revert 161821 "Remove 'FAST_MOBILE_SCROLLING'"

Source/core/rendering/RenderBoxModelObject.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2005 Allan Sandfeld Jensen (kde@carewolf.com)
  *           (C) 2005, 2006 Samuel Weinig (sam.weinig@gmail.com)
  * Copyright (C) 2005, 2006, 2007, 2008, 2009 Apple Inc. All rights reserved.
  * Copyright (C) 2010 Google Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
@@ skipping 989 matching lines @@
     BackgroundImageGeometry& geometry, RenderObject* backgroundObject)
 {
     LayoutUnit left = 0;
     LayoutUnit top = 0;
     IntSize positioningAreaSize;
     IntRect snappedPaintRect = pixelSnappedIntRect(paintRect);
 
     // Determine the background positioning area and set destRect to the background painting area.
     // destRect will be adjusted later if the background is non-repeating.
     bool fixedAttachment = fillLayer->attachment() == FixedBackgroundAttachment;
+
+#if ENABLE(FAST_MOBILE_SCROLLING)
+    if (view()->frameView() && view()->frameView()->canBlitOnScroll()) {
+        // As a side effect of an optimization to blit on scroll, we do not honor the CSS
+        // property "background-attachment: fixed" because it may result in rendering
+        // artifacts. Note, these artifacts only appear if we are blitting on scroll of
+        // a page that has fixed background images.
+        fixedAttachment = false;
+    }
+#endif
+
     if (!fixedAttachment) {
         geometry.setDestRect(snappedPaintRect);
 
         LayoutUnit right = 0;
         LayoutUnit bottom = 0;
         // Scroll and Local.
         if (fillLayer->origin() != BorderFillBox) {
             left = borderLeft();
             right = borderRight();
             top = borderTop();
@@ skipping 1790 matching lines @@
     ASSERT(!beforeChild || toBoxModelObject == beforeChild->parent());
     for (RenderObject* child = startChild; child && child != endChild; ) {
         // Save our next sibling as moveChildTo will clear it.
         RenderObject* nextSibling = child->nextSibling();
         moveChildTo(toBoxModelObject, child, beforeChild, fullRemoveInsert);
         child = nextSibling;
     }
 }
 
 } // namespace WebCore