Bug 1459997 - Add much more rigorous assertions for retained-dl assumptions. r=miko

This adds an assertion checking for duplicate items whenever we create an item, and when we merge an item into the final list.

I had to disable tracking for the anonymous inner list for nsDisplayPerspective and nsDisplayTransform (and manually forward RemoveFrame to them), as well as skipping the assertion for multi-page (since we can end up duplicating wrap lists, but isn't a problem, since we don't retain these).

MozReview-Commit-ID: 4n6rx9bQNan

layout/generic/nsSimplePageSequenceFrame.cpp

@@ -720,6 +720,8 @@ void
 nsSimplePageSequenceFrame::BuildDisplayList(nsDisplayListBuilder*   aBuilder,
                                             const nsDisplayListSet& aLists)
 {
+  aBuilder->SetInPageSequence(true);
+  aBuilder->SetDisablePartialUpdates(true);
   DisplayBorderBackgroundOutline(aBuilder, aLists);
 
   nsDisplayList content;
@@ -753,6 +755,7 @@ nsSimplePageSequenceFrame::BuildDisplayList(nsDisplayListBuilder*   aBuilder,
                                           ::ComputePageSequenceTransform));
 
   aLists.Content()->AppendToTop(&content);
+  aBuilder->SetInPageSequence(false);
 }
 
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