cc: Preserve currently scrolling layer during tree activation

cc/layer_tree_impl.h

Index: cc/layer_tree_impl.h
diff --git a/cc/layer_tree_impl.h b/cc/layer_tree_impl.h
index fef94c551b0c68e446f2cd08684f2e4aa4dd7104..801dde6ba6e33aac9b55f777ab9b4861d2c4a1e3 100644
--- a/cc/layer_tree_impl.h
+++ b/cc/layer_tree_impl.h
@@ -129,6 +129,8 @@ class CC_EXPORT LayerTreeImpl {
 
   AnimationRegistrar* animationRegistrar() const;
 
+  void PushPersistedState(LayerTreeImpl* pendingTree);
+
 protected:
   LayerTreeImpl(LayerTreeHostImpl* layer_tree_host_impl);
 
@@ -144,7 +146,7 @@ protected:
   typedef base::hash_map<int, LayerImpl*> LayerIdMap;
   LayerIdMap layer_id_map_;
 
-  // Persisted state
+  // Persisted state for non-impl-side-painting.
   int scrolling_layer_id_from_previous_tree_;
 
   // List of visible layers for the most recently prepared frame. Used for