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

@@ -1221,6 +1221,7 @@ nsMenuPopupFrame::SetPopupPosition(nsIFrame* aAnchorFrame, bool aIsMove, bool aS
   // Relative to the screen
   parentRect.MoveBy(referenceFrame->GetScreenRectInAppUnits().TopLeft());
   // In its own app units
+  parentRect =
     parentRect.ConvertAppUnitsRoundOut(rootPresContext->AppUnitsPerDevPixel(),
                                        presContext->AppUnitsPerDevPixel());