Bug 1679206 - Use nsPlaceholderFrame::GetRealFrameForPlaceholder in ViewportFrame.cpp r=emilio

Differential Revision: https://phabricator.services.mozilla.com/D97936

layout/generic/ViewportFrame.cpp

@@ -182,13 +182,11 @@ void ViewportFrame::BuildDisplayListForTopLayer(nsDisplayListBuilder* aBuilder,
       }
       if (nsIFrame* backdropPh =
               frame->GetChildList(kBackdropList).FirstChild()) {
-        MOZ_ASSERT(backdropPh->IsPlaceholderFrame());
         MOZ_ASSERT(!backdropPh->GetNextSibling(), "more than one ::backdrop?");
         MOZ_ASSERT(backdropPh->HasAnyStateBits(NS_FRAME_FIRST_REFLOW),
                    "did you intend to reflow ::backdrop placeholders?");
         nsIFrame* backdropFrame =
-            static_cast<nsPlaceholderFrame*>(backdropPh)->GetOutOfFlowFrame();
-        MOZ_ASSERT(backdropFrame);
+            nsPlaceholderFrame::GetRealFrameForPlaceholder(backdropPh);
         BuildDisplayListForTopLayerFrame(aBuilder, backdropFrame, aList);
 
         if (aIsOpaque) {