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 33a9f6dea9269f07fafe4579ee7e87dadc97bb8b..9b676a52e0039322d22a38f6a8e4b3034d26804b 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;
@@ -957,6 +958,7 @@ private:
 
     // For Slimming Paint v2 only.
     std::unique_ptr<PaintController> m_paintController;
+    std::unique_ptr<PaintArtifactCompositor> m_paintArtifactCompositor;
 };
 
 inline void FrameView::incrementVisuallyNonEmptyCharacterCount(unsigned count)