Begin tracking why layout is invalidated

Source/core/layout/LayoutBlockFlow.cpp

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 1316 matching lines @@
         setLogicalHeight(logicalTop);
         if (!previousBlockFlow->avoidsFloats() && (previousBlockFlow->logicalTop() + previousBlockFlow->lowestFloatLogicalBottom()) > logicalTop)
             addOverhangingFloats(previousBlockFlow, false);
         setLogicalHeight(oldLogicalHeight);
 
         // If |child|'s previous sibling is a self-collapsing block that cleared a float and margin collapsing resulted in |child| moving up
         // into the margin area of the self-collapsing block then the float it clears is now intruding into |child|. Layout again so that we can look for
         // 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;
 }
 
 void LayoutBlockFlow::adjustPositionedBlock(LayoutBox& child, const MarginInfo& marginInfo)
 {
     bool isHorizontal = isHorizontalWritingMode();
     bool hasStaticBlockPosition = child.style()->hasStaticBlockPosition(isHorizontal);
 
@@ skipping 1814 matching lines @@
     }
     if (isAnonymous())
         return "LayoutBlockFlow (anonymous)";
     if (isRelPositioned())
         return "LayoutBlockFlow (relative positioned)";
     return "LayoutBlockFlow";
 }
 
 
 } // namespace blink