Bug 819837 - Always treat empty images as being background-attachment:scroll, r=roc

layout/base/nsCSSRendering.cpp

@@ -2845,7 +2845,8 @@ nsCSSRendering::ComputeBackgroundPositioningArea(nsPresContext* aPresContext,
   }
 
   nsIFrame* attachedToFrame = aForFrame;
-  if (NS_STYLE_BG_ATTACHMENT_FIXED == aLayer.mAttachment) {
+  if (NS_STYLE_BG_ATTACHMENT_FIXED == aLayer.mAttachment &&
+      !aLayer.mImage.IsEmpty()) {
     // If it's a fixed background attachment, then the image is placed
     // relative to the viewport, which is the area of the root frame
     // in a screen context or the page content frame in a print context.