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 3b186932663ae4f585d5cb46cf41603e094605c5..c2859e1db95706adca0eac533671a84b2daee38b 100644
--- a/cc/trees/layer_tree_impl.h
+++ b/cc/trees/layer_tree_impl.h
@@ -117,6 +117,9 @@ class CC_EXPORT LayerTreeImpl {
 
   void FindRootScrollLayer();
   void UpdateMaxScrollOffset();
+  void SetPinchViewportLayerIds(int page_scale_layer_id,
+                                int inner_viewport_scroll_layer_id,
+                                int outer_viewport_scroll_layer_id);
   void ApplySentScrollAndScaleDeltasFromAbortedCommit();
   void ApplyScrollDeltasSinceBeginFrame();
 
@@ -226,6 +229,10 @@ 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_;