Plumb PinchVirtualViewport layers into CC.

cc/trees/layer_tree_impl.h

Index: cc/trees/layer_tree_impl.h
diff --git a/cc/trees/layer_tree_impl.h b/cc/trees/layer_tree_impl.h
index 969cba2d0756a3e77da8749175dd4c02d51abf1a..027235e641045b02ffaee474fc7e5b072dba6352 100644
--- a/cc/trees/layer_tree_impl.h
+++ b/cc/trees/layer_tree_impl.h
@@ -118,6 +118,10 @@ class CC_EXPORT LayerTreeImpl {
 
   void FindRootScrollLayer();
   void UpdateMaxScrollOffset();
+  void SetViewportLayersFromIds(int page_scale_layer_id,
+                                int inner_viewport_scroll_layer_id,
+                                int outer_viewport_scroll_layer_id);
+  void ClearViewportLayers();
   void ApplySentScrollAndScaleDeltasFromAbortedCommit();
   void ApplyScrollDeltasSinceBeginFrame();
 
@@ -227,6 +231,10 @@ class CC_EXPORT LayerTreeImpl {
   SkColor background_color_;
   bool has_transparent_background_;
 
+  LayerImpl* page_scale_layer_;
+  LayerImpl* inner_viewport_scroll_layer_;
+  LayerImpl* outer_viewport_scroll_layer_;
+
   float page_scale_factor_;
   float page_scale_delta_;
   float sent_page_scale_delta_;