Implement the algorithm to test merging and overlap in PaintArtifactCompositor.

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

Index: third_party/WebKit/Source/core/paint/PaintPropertyTreeBuilder.h
diff --git a/third_party/WebKit/Source/core/paint/PaintPropertyTreeBuilder.h b/third_party/WebKit/Source/core/paint/PaintPropertyTreeBuilder.h
index 350073f43cf0682d3f919badd70ad5b98878da83..9ce2621549aed16dd23a85d4a1598b12277cfa6a 100644
--- a/third_party/WebKit/Source/core/paint/PaintPropertyTreeBuilder.h
+++ b/third_party/WebKit/Source/core/paint/PaintPropertyTreeBuilder.h
@@ -14,6 +14,7 @@
 
 namespace blink {
 
+class CompositingReasonFinder;
 class FrameView;
 class LayoutObject;
 
@@ -113,8 +114,7 @@ class PaintPropertyTreeBuilder {
  private:
   static void updatePaintOffsetTranslation(const LayoutObject&,
                                            PaintPropertyTreeBuilderContext&);
-  static void updateTransform(const LayoutObject&,
-                              PaintPropertyTreeBuilderContext&);
+  void updateTransform(const LayoutObject&, PaintPropertyTreeBuilderContext&);
   static void updateTransformForNonRootSVG(const LayoutObject&,
                                            PaintPropertyTreeBuilderContext&);
   static void updateEffect(const LayoutObject&,
@@ -137,6 +137,8 @@ class PaintPropertyTreeBuilder {
       PaintPropertyTreeBuilderContext&);
   static void updateOutOfFlowContext(const LayoutObject&,
                                      PaintPropertyTreeBuilderContext&);
+
+  std::unique_ptr<CompositingReasonFinder> m_compositingReasonFinder;
 };
 
 }  // namespace blink