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..ab8f89978d2dde4e5778c4d8ddb074fc285d108b 100644
--- a/third_party/WebKit/Source/platform/graphics/compositing/PaintArtifactCompositor.h
+++ b/third_party/WebKit/Source/platform/graphics/compositing/PaintArtifactCompositor.h
@@ -76,9 +76,14 @@ private:
     // return the actual origin of the paint chunk in output parameter layerOffset.
     scoped_refptr<cc::Layer> layerForPaintChunk(const PaintArtifact&, const PaintChunk&, gfx::Vector2dF& layerOffset);
 
+    // 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;
-    Vector<std::unique_ptr<ContentLayerClientImpl>> m_contentLayerClients;
+    Vector<std::unique_ptr<ContentLayerClientImpl>> m_currentContentLayerClients;
+    Vector<std::unique_ptr<ContentLayerClientImpl>> m_newContentLayerClients;
 
     bool m_extraDataForTestingEnabled = false;
     std::unique_ptr<ExtraDataForTesting> m_extraDataForTesting;