Begin tracking why layout is invalidated

Source/core/layout/LayoutBlockFlow.cpp

Index: Source/core/layout/LayoutBlockFlow.cpp
diff --git a/Source/core/layout/LayoutBlockFlow.cpp b/Source/core/layout/LayoutBlockFlow.cpp
index b30512e4162fcd4c92af31d5192d0ae1f684e152..de129a19220997a57509879bdcd4bb92f662629e 100644
--- a/Source/core/layout/LayoutBlockFlow.cpp
+++ b/Source/core/layout/LayoutBlockFlow.cpp
@@ -1334,7 +1334,7 @@ LayoutUnit LayoutBlockFlow::collapseMargins(LayoutBox& child, MarginInfo& margin
         // floats in the parent that overhang |child|'s new logical top.
         bool logicalTopIntrudesIntoFloat = clearanceForSelfCollapsingBlock > 0 && logicalTop < beforeCollapseLogicalTop;
         if (logicalTopIntrudesIntoFloat && containsFloats() && !child.avoidsFloats() && lowestFloatLogicalBottom() > logicalTop)
-            child.setNeedsLayoutAndFullPaintInvalidation();
+            child.setNeedsLayoutAndFullPaintInvalidation(LayoutInvalidationReason::AncestorMarginCollapsing);
     }
 
     return logicalTop;