Bug 1143974. When converting the anchor rect for a xul popup make sure to actually convert for app units differences. r=mats

ConvertAppUnitsRoundOut returns the result, it doesn't modify |this|.

layout/xul/nsMenuPopupFrame.cpp

@@ -1221,8 +1221,9 @@ nsMenuPopupFrame::SetPopupPosition(nsIFrame* aAnchorFrame, bool aIsMove, bool aS
   // Relative to the screen
   parentRect.MoveBy(referenceFrame->GetScreenRectInAppUnits().TopLeft());
   // In its own app units
-  parentRect.ConvertAppUnitsRoundOut(rootPresContext->AppUnitsPerDevPixel(),
-                                     presContext->AppUnitsPerDevPixel());
+  parentRect =
+    parentRect.ConvertAppUnitsRoundOut(rootPresContext->AppUnitsPerDevPixel(),
+                                       presContext->AppUnitsPerDevPixel());
 
   // Set the popup's size to the preferred size. Below, this size will be
   // adjusted to fit on the screen or within the content area. If the anchor