Implement the algorithm to test merging and overlap in PaintArtifactCompositor.

third_party/WebKit/Source/core/paint/PaintPropertyTreeBuilder.cpp

Index: third_party/WebKit/Source/core/paint/PaintPropertyTreeBuilder.cpp
diff --git a/third_party/WebKit/Source/core/paint/PaintPropertyTreeBuilder.cpp b/third_party/WebKit/Source/core/paint/PaintPropertyTreeBuilder.cpp
index a0a17d3962ef13e7d138575fc7b25be600a0f5ca..c62a1e9af3837e45b579460999d6eb70e4bb54be 100644
--- a/third_party/WebKit/Source/core/paint/PaintPropertyTreeBuilder.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintPropertyTreeBuilder.cpp
@@ -9,6 +9,7 @@
 #include "core/frame/Settings.h"
 #include "core/layout/LayoutInline.h"
 #include "core/layout/LayoutView.h"
+#include "core/layout/compositing/CompositingReasonFinder.h"
 #include "core/layout/svg/LayoutSVGRoot.h"
 #include "core/paint/FindPropertiesNeedingUpdate.h"
 #include "core/paint/ObjectPaintProperties.h"
@@ -122,6 +123,9 @@ void PaintPropertyTreeBuilder::updateProperties(
     return;
   }
 
+  m_compositingReasonFinder.reset(
+      new CompositingReasonFinder(*frameView.layoutView()));
+
 #if DCHECK_IS_ON()
   FindFrameViewPropertiesNeedingUpdateScope checkScope(&frameView, context);
 #endif
@@ -338,7 +342,10 @@ void PaintPropertyTreeBuilder::updateTransform(
       context.forceSubtreeUpdate |= properties.updateTransform(
           context.current.transform, matrix,
           transformOrigin(toLayoutBox(object)),
-          context.current.shouldFlattenInheritedTransform, renderingContextID);
+          context.current.shouldFlattenInheritedTransform, renderingContextID,
+          m_compositingReasonFinder->directReasons(
+              toLayoutBoxModelObject(&object)->layer()) &
+              CompositingReason3DTransform);
     } else {
       if (auto* properties = object.getMutableForPainting().paintProperties())
         context.forceSubtreeUpdate |= properties->clearTransform();