Refactor the way PaintArtifactCompositor is attached and used.

third_party/WebKit/Source/web/WebFrameWidgetBase.h

Index: third_party/WebKit/Source/web/WebFrameWidgetBase.h
diff --git a/third_party/WebKit/Source/web/WebFrameWidgetBase.h b/third_party/WebKit/Source/web/WebFrameWidgetBase.h
index e71ac932cd98197433eb25e0ecd953aaa40f6940..1609eeae4e4a11ebe9d95908957dbaa6a6401fdd 100644
--- a/third_party/WebKit/Source/web/WebFrameWidgetBase.h
+++ b/third_party/WebKit/Source/web/WebFrameWidgetBase.h
@@ -13,6 +13,7 @@ namespace blink {
 class CompositorAnimationTimeline;
 class CompositorProxyClient;
 class GraphicsLayer;
+class WebLayer;
 
 class WebFrameWidgetBase : public WebFrameWidget {
 public:
@@ -25,6 +26,9 @@ public:
     // the root layer.
     virtual void setRootGraphicsLayer(GraphicsLayer*) = 0;
 
+    // Sets the root layer. |WebLayer| can be null when detaching the root layer.
+    virtual void setRootLayer(WebLayer*) = 0;
+
     // Attaches/detaches a CompositorAnimationTimeline to the layer tree.
     virtual void attachCompositorAnimationTimeline(CompositorAnimationTimeline*) = 0;
     virtual void detachCompositorAnimationTimeline(CompositorAnimationTimeline*) = 0;