Add a cache of LayerImpl's viewport layer type

cc/trees/layer_tree_impl_unittest.cc

Index: cc/trees/layer_tree_impl_unittest.cc
diff --git a/cc/trees/layer_tree_impl_unittest.cc b/cc/trees/layer_tree_impl_unittest.cc
index 089c2fb9b4009c490737ce14fbb5d0e2e6bec301..5d3a3de07f6ca3eb72a86ce30d3b31136ab746b1 100644
--- a/cc/trees/layer_tree_impl_unittest.cc
+++ b/cc/trees/layer_tree_impl_unittest.cc
@@ -2058,7 +2058,8 @@ TEST_F(LayerTreeImplTest, SelectionBoundsForScaledLayers) {
       root->bounds(), device_scale_factor * page_scale_factor);
   host_impl().SetViewportSize(scaled_bounds_for_root);
 
-  host_impl().active_tree()->SetViewportLayersFromIds(0, root->id(), 0, 0);
+  host_impl().active_tree()->SetViewportLayersFromIds(
+      Layer::INVALID_ID, root->id(), Layer::INVALID_ID, Layer::INVALID_ID);
   host_impl().active_tree()->SetDeviceScaleFactor(device_scale_factor);
   host_impl().active_tree()->SetPageScaleOnActiveTree(page_scale_factor);
   host_impl().active_tree()->SetViewportLayersFromIds(Layer::INVALID_ID, 1, 1,