Reuse cc::PictureLayers when possible for SPv2.

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 34b6e2f9f104ae073f907d5b8bce778bf9b087af..09471226ed730b1212a0a63f8f678560277cc325 100644
--- a/third_party/WebKit/Source/platform/graphics/compositing/PaintArtifactCompositor.h
+++ b/third_party/WebKit/Source/platform/graphics/compositing/PaintArtifactCompositor.h
@@ -74,7 +74,12 @@ private:
     // 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 output parameter layerOffset.
-    scoped_refptr<cc::Layer> layerForPaintChunk(const PaintArtifact&, const PaintChunk&, gfx::Vector2dF& layerOffset);
+    scoped_refptr<cc::Layer> layerForPaintChunk(const PaintArtifact&, const PaintChunk&, gfx::Vector2dF& layerOffset,
+        Vector<std::unique_ptr<ContentLayerClientImpl>>& newContentLayerClients);
+
+    // Finds a client among the current vector of clients that matches the paint chunk's id,
+    // or otherwise allocates a new one.
+    std::unique_ptr<ContentLayerClientImpl> clientForPaintChunk(const PaintChunk&);
 
     scoped_refptr<cc::Layer> m_rootLayer;
     std::unique_ptr<WebLayer> m_webLayer;