Revert 161821 "Remove 'FAST_MOBILE_SCROLLING'"

Source/core/rendering/RenderObject.cpp

Index: Source/core/rendering/RenderObject.cpp
diff --git a/Source/core/rendering/RenderObject.cpp b/Source/core/rendering/RenderObject.cpp
index 9e96c62fc87aacd6c15321481e24bd16b9e02627..e5e485f2d7489c680686665c201a32d9583f992d 100644
--- a/Source/core/rendering/RenderObject.cpp
+++ b/Source/core/rendering/RenderObject.cpp
@@ -2027,11 +2027,20 @@ void RenderObject::styleWillChange(StyleDifference diff, const RenderStyle* newS
         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;