Add a cache of LayerImpl's viewport layer type

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 6f1783cc0fae84ec515966a7ab1c79121d999660..05e54add9ff4d617bc0f5062f6bce38efc75ab54 100644
--- a/cc/trees/layer_tree_impl.h
+++ b/cc/trees/layer_tree_impl.h
@@ -189,6 +189,9 @@ class CC_EXPORT LayerTreeImpl {
                                 int inner_viewport_scroll_layer_id,
                                 int outer_viewport_scroll_layer_id);
   void ClearViewportLayers();
+  // Sets the viewport layer types. This depends on the viewport layers from
+  // SetViewportLayersFromIds and the viewport layer's scroll_clip_layer_ids.
+  void UpdateViewportLayerTypes();
   LayerImpl* OverscrollElasticityLayer() {
     return LayerById(overscroll_elasticity_layer_id_);
   }