Revert 161821 "Remove 'FAST_MOBILE_SCROLLING'"

Source/core/rendering/RenderBoxModelObject.cpp

Index: Source/core/rendering/RenderBoxModelObject.cpp
diff --git a/Source/core/rendering/RenderBoxModelObject.cpp b/Source/core/rendering/RenderBoxModelObject.cpp
index 4e2bfec0a8e614ec65fac32d5345672734620a26..caef81861c47db979ac5417313a251fb3679192e 100644
--- a/Source/core/rendering/RenderBoxModelObject.cpp
+++ b/Source/core/rendering/RenderBoxModelObject.cpp
@@ -1007,6 +1007,17 @@ void RenderBoxModelObject::calculateBackgroundImageGeometry(const FillLayer* fil
     // 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);