Revert 161821 "Remove 'FAST_MOBILE_SCROLLING'"

Source/core/rendering/RenderObject.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2000 Dirk Mueller (mueller@kde.org)
  *           (C) 2004 Allan Sandfeld Jensen (kde@carewolf.com)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2009 Google Inc. All rights reserved.
  * Copyright (C) 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  *
  * This library is free software; you can redistribute it and/or
@@ skipping 2009 matching lines @@
         // Clearing these bits is required to avoid leaving stale renderers.
         // FIXME: We shouldn't need that hack if our logic was totally correct.
         if (diff == StyleDifferenceLayout || diff == StyleDifferenceLayoutPositionedMovementOnly) {
             setFloating(false);
             clearPositionedState();
         }
     } else
         s_affectsParentBlock = false;
 
     if (view()->frameView()) {
-        bool newStyleSlowScroll = newStyle && newStyle->hasFixedBackgroundImage();
-        bool oldStyleSlowScroll = m_style && m_style->hasFixedBackgroundImage();
+        bool shouldBlitOnFixedBackgroundImage = false;
+#if ENABLE(FAST_MOBILE_SCROLLING)
+        // On low-powered/mobile devices, preventing blitting on a scroll can cause noticeable delays
+        // when scrolling a page with a fixed background image. As an optimization, assuming there are
+        // no fixed positoned elements on the page, we can acclerate scrolling (via blitting) if we
+        // ignore the CSS property "background-attachment: fixed".
+        shouldBlitOnFixedBackgroundImage = true;
+#endif
+
+        bool newStyleSlowScroll = newStyle && !shouldBlitOnFixedBackgroundImage && newStyle->hasFixedBackgroundImage();
+        bool oldStyleSlowScroll = m_style && !shouldBlitOnFixedBackgroundImage && m_style->hasFixedBackgroundImage();
 
         bool drawsRootBackground = isRoot() || (isBody() && !rendererHasBackground(document().documentElement()->renderer()));
-        if (drawsRootBackground) {
+        if (drawsRootBackground && !shouldBlitOnFixedBackgroundImage) {
             if (view()->compositor()->supportsFixedRootBackgroundCompositing()) {
                 if (newStyleSlowScroll && newStyle->hasEntirelyFixedBackground())
                     newStyleSlowScroll = false;
 
                 if (oldStyleSlowScroll && m_style->hasEntirelyFixedBackground())
                     oldStyleSlowScroll = false;
             }
         }
 
         if (oldStyleSlowScroll != newStyleSlowScroll) {
@@ skipping 1346 matching lines @@
 {
     if (object1) {
         const WebCore::RenderObject* root = object1;
         while (root->parent())
             root = root->parent();
         root->showRenderTreeAndMark(object1, "*", object2, "-", 0);
     }
 }
 
 #endif