Create Pinch Virtual Viewport scrollbar layers in Blink.

public/platform/WebLayerTreeView.h

Index: public/platform/WebLayerTreeView.h
diff --git a/public/platform/WebLayerTreeView.h b/public/platform/WebLayerTreeView.h
index 0c67c2e5f720df158a132d7e92b1c5d62eae4c0d..a5d1aa6f6b43468b3d9155b19a76af449381254e 100644
--- a/public/platform/WebLayerTreeView.h
+++ b/public/platform/WebLayerTreeView.h
@@ -122,14 +122,11 @@ public:
     virtual void registerForAnimations(WebLayer* layer) { }
 
     // Identify key layers to the compositor when using the pinch virtual viewport.
-    virtual void registerPinchViewportLayers(
-        const WebLayer* innerViewportContainerLayer,
+    virtual void registerViewportLayers(
         const WebLayer* pageScaleLayerLayer,
         const WebLayer* innerViewportScrollLayer,
-        const WebLayer* outerViewportScrollLayer,
-        const WebLayer* innerViewportHorizontalScrollbarLayer,
-        const WebLayer* innerViewportVerticalScrollbarLayer) { }
-    virtual void clearPinchViewportLayers() { }
+        const WebLayer* outerViewportScrollLayer) { }
+    virtual void clearViewportLayers() { }
 
     // Debugging / dangerous ---------------------------------------------