Insert pinch zoom virtual viewport layers to graphics layer tree.

public/platform/WebLayerTreeView.h

Index: public/platform/WebLayerTreeView.h
diff --git a/public/platform/WebLayerTreeView.h b/public/platform/WebLayerTreeView.h
index 99d9ec5ec77ffbcc3aa9127d78f42f622f91f07f..e40d22c39746e1f5051af851c87eec69cac548fa 100644
--- a/public/platform/WebLayerTreeView.h
+++ b/public/platform/WebLayerTreeView.h
@@ -121,6 +121,15 @@ public:
     // been added to the tree.
     virtual void registerForAnimations(WebLayer* layer) { }
 
+    // Identify key layers to the compositor when using the pinch virtual viewport.
+    virtual void registerPinchViewportLayers(
+        const WebLayer* innerViewportClipLayer,
+        const WebLayer* pageScaleLayerLayer,
+        const WebLayer* innerViewportScrollLayer,
+        const WebLayer* outerViewportScrollLayer,
+        const WebLayer* innerViewportHorizontalScrollbarLayer,
+        const WebLayer* innerViewportVerticalScrollbarLayer) { }
+    virtual void clearPinchViewportLayers() { }
 
     // Debugging / dangerous ---------------------------------------------