Refactor the way PaintArtifactCompositor is attached and used.

third_party/WebKit/Source/core/frame/FrameView.h

Index: third_party/WebKit/Source/core/frame/FrameView.h
diff --git a/third_party/WebKit/Source/core/frame/FrameView.h b/third_party/WebKit/Source/core/frame/FrameView.h
index 60a2251c233a8e87209409ce88c86448c5b71ac1..f0e77cf8d90a9d43a69c5ae40e7945e12d0fa3f1 100644
--- a/third_party/WebKit/Source/core/frame/FrameView.h
+++ b/third_party/WebKit/Source/core/frame/FrameView.h
@@ -79,6 +79,7 @@ class LayoutObject;
 class LayoutReplaced;
 class LayoutScrollbarPart;
 class LayoutView;
+class PaintArtifactCompositor;
 class PaintController;
 class PaintInvalidationState;
 class Page;
@@ -953,6 +954,7 @@ private:
 
     // For Slimming Paint v2 only.
     std::unique_ptr<PaintController> m_paintController;
+    std::unique_ptr<PaintArtifactCompositor> m_paintArtifactCompositor;
 };
 
 inline void FrameView::incrementVisuallyNonEmptyCharacterCount(unsigned count)