Implement the algorithm to test merging and overlap in PaintArtifactCompositor.

third_party/WebKit/Source/platform/graphics/compositing/PaintArtifactCompositor.h

Index: third_party/WebKit/Source/platform/graphics/compositing/PaintArtifactCompositor.h
diff --git a/third_party/WebKit/Source/platform/graphics/compositing/PaintArtifactCompositor.h b/third_party/WebKit/Source/platform/graphics/compositing/PaintArtifactCompositor.h
index 6f7c2bc57854c878a0f1199276af4290759d7f50..f1cb5b3de66d4a0a64aab1f63d6411977a5a2567 100644
--- a/third_party/WebKit/Source/platform/graphics/compositing/PaintArtifactCompositor.h
+++ b/third_party/WebKit/Source/platform/graphics/compositing/PaintArtifactCompositor.h
@@ -26,10 +26,27 @@ class Vector2dF;
 namespace blink {
 
 class JSONObject;
+class LayoutView;
 class PaintArtifact;
 class WebLayer;
 struct PaintChunk;
 
+struct PendingLayer {
+  PendingLayer(const PaintChunk& firstPaintChunk) : knownToBeOpaque(false) {
+    add(firstPaintChunk);
+  }
+
+  void add(const PaintChunk& paintChunk) {
+    paintChunks.append(paintChunk);
+    bounds.unite(paintChunk.bounds);
+    knownToBeOpaque = knownToBeOpaque && paintChunk.knownToBeOpaque;
+  }
+
+  FloatRect bounds;
+  Vector<PaintChunk> paintChunks;
+  bool knownToBeOpaque;
+};
+
 // Responsible for managing compositing in terms of a PaintArtifact.
 //
 // Owns a subtree of the compositor layer tree, and updates it in response to
@@ -82,18 +99,21 @@ class PLATFORM_EXPORT PaintArtifactCompositor {
 
   class ContentLayerClientImpl;
 
+  void collectPendingLayers(const Vector<PaintChunk>& paintChunks,
+                            Vector<PendingLayer>& pendingLayers);
+
   // Builds a leaf layer that represents a single paint chunk.
   // Note: cc::Layer API assumes the layer bounds start at (0, 0), but the
   // bounding box of a paint chunk does not necessarily start at (0, 0) (and
   // could even be negative). Internally the generated layer translates the
   // paint chunk to align the bounding box to (0, 0) and return the actual
   // origin of the paint chunk in the |layerOffset| outparam.
-  scoped_refptr<cc::Layer> layerForPaintChunk(
+  scoped_refptr<cc::Layer> compositedLayerForPendingLayer(
       const PaintArtifact&,
-      const PaintChunk&,
+      const PendingLayer&,
       gfx::Vector2dF& layerOffset,
       Vector<std::unique_ptr<ContentLayerClientImpl>>& newContentLayerClients,
-      RasterInvalidationTracking*);
+      RasterInvalidationTrackingMap<const PaintChunk>*);
 
   // Finds a client among the current vector of clients that matches the paint
   // chunk's id, or otherwise allocates a new one.