Implement the algorithm to test merging and overlap in PaintArtifactCompositor.

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

Index: third_party/WebKit/Source/core/paint/PaintLayerPainter.cpp
diff --git a/third_party/WebKit/Source/core/paint/PaintLayerPainter.cpp b/third_party/WebKit/Source/core/paint/PaintLayerPainter.cpp
index 18d5d0ad0ed5dbf9f8bf0312067f0a7ce6c9e8ec..a330cb592343bff56ecce39f56e7bec12c25a278 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayerPainter.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintLayerPainter.cpp
@@ -276,6 +276,9 @@ PaintResult PaintLayerPainter::paintLayerContents(
     properties.effect = localBorderBoxProperties.propertyTreeState.effect();
     properties.backfaceHidden =
         m_paintLayer.layoutObject()->hasHiddenBackface();
+    properties.willChangeTransform =
+        m_paintLayer.layoutObject()->styleRef().hasWillChangeCompositingHint();
+
     scopedPaintChunkProperties.emplace(context.getPaintController(),
                                        m_paintLayer, properties);
   }
@@ -488,6 +491,10 @@ PaintResult PaintLayerPainter::paintLayerContents(
       properties.effect = localBorderBoxProperties.propertyTreeState.effect();
       properties.backfaceHidden =
           m_paintLayer.layoutObject()->hasHiddenBackface();
+      properties.willChangeTransform = m_paintLayer.layoutObject()

[trchen, 2016/12/13 22:46:01]

I think this is not quite right. For example:

<div style="will-change:transform;">
  <div style="position:absolute; z-index:0; overflow:hidden;">Foo</div>
</div>

The "Foo" element should be promoted to a cc layer for the will-change hint, but
this code won't propagate the compositing hint from its parent.

Also simple propagation won't work, for example:

<div style="will-change:transform;">
  <div style="position:absolute; z-index:0; overflow:hidden;">Foo</div>
  <div style="position:absolute; z-index:0; overflow:hidden;">Bar</div>
</div>

Both 'Foo' and 'Bar' want a cc layer other than their grand parent (not shown in
the example), but they certainly can share a layer.

This is why in your doc I said I'm not certain about the concept of "chunk
compositing reason". The only thing I can think of is foreign layer chunk. Other
than that all compositing reasons are from property nodes.

I would model it this way:
A transform node with 3D transform, will-change hint, animation, scrolling, blah
blah blah gets direct compositing reason. A transform node that has direct
compositing reason mark its parent edge. Two chunks can't be merged if the path
between their transform nodes contains marked edge.

+                                           ->styleRef()
+                                           .hasWillChangeCompositingHint();
+
       contentScopedPaintChunkProperties.emplace(context.getPaintController(),
                                                 m_paintLayer, properties);
     }