Implement the algorithm to test merging and overlap in PaintArtifactCompositor.

third_party/WebKit/Source/platform/graphics/paint/PaintChunker.cpp

Index: third_party/WebKit/Source/platform/graphics/paint/PaintChunker.cpp
diff --git a/third_party/WebKit/Source/platform/graphics/paint/PaintChunker.cpp b/third_party/WebKit/Source/platform/graphics/paint/PaintChunker.cpp
index c4f087aa853f5d02c1edbc12f71591d18fe7d2d3..a2b3bfdc71e59f29336abfa0c33c114d0d0f5e6f 100644
--- a/third_party/WebKit/Source/platform/graphics/paint/PaintChunker.cpp
+++ b/third_party/WebKit/Source/platform/graphics/paint/PaintChunker.cpp
@@ -14,7 +14,7 @@ DisableNullPaintPropertyChecks::DisableNullPaintPropertyChecks()
     : m_disabler(&gNullPaintPropertyChecksDisabled, true) {}
 #endif
 
-PaintChunker::PaintChunker() {}
+PaintChunker::PaintChunker() : m_nextDisplayItemBehavior(DefaultBehavior) {}
 
 PaintChunker::~PaintChunker() {}
 
@@ -26,6 +26,13 @@ void PaintChunker::updateCurrentPaintChunkProperties(
   m_currentChunkId = WTF::nullopt;
   if (chunkId)
     m_currentChunkId.emplace(*chunkId);
+
+  // TODO(chrishtr): why not just compare properties != m_currentProperties?
+  // It seems this would solve the "are the property tree states the same"?
+  if (properties.backfaceHidden != m_currentProperties.backfaceHidden ||
+      properties.willChangeTransform != m_currentProperties.willChangeTransform)
+    m_nextDisplayItemBehavior = RequiresSeparateChunk;
+
   m_currentProperties = properties;
 }
 
@@ -47,7 +54,11 @@ bool PaintChunker::incrementDisplayItemIndex(const DisplayItem& item) {
 
   ItemBehavior behavior;
   Optional<PaintChunk::Id> newChunkId;
-  if (DisplayItem::isForeignLayerType(item.getType())) {
+  if (m_nextDisplayItemBehavior == RequiresSeparateChunk) {
+    behavior = RequiresSeparateChunk;
+    if (!item.skippedCache() && m_currentChunkId)
+      newChunkId.emplace(*m_currentChunkId);
+  } else if (DisplayItem::isForeignLayerType(item.getType())) {
     behavior = RequiresSeparateChunk;
     // Use null chunkId if we are skipping cache, so that the chunk will not
     // match any old chunk and will be treated as brand new.