change overlay scrollbar hover show to hover fade in

cc/trees/layer_tree_impl.cc

Index: cc/trees/layer_tree_impl.cc
diff --git a/cc/trees/layer_tree_impl.cc b/cc/trees/layer_tree_impl.cc
index f7f0304ee5f74064c75be4de70aa691da4cd3487..3a76044430462bd30b5773887af5b7035939c988 100644
--- a/cc/trees/layer_tree_impl.cc
+++ b/cc/trees/layer_tree_impl.cc
@@ -1485,14 +1485,16 @@ LayerTreeImpl::CreateScrollbarAnimationController(int scroll_layer_id) {
     }
     case LayerTreeSettings::AURA_OVERLAY: {
       DCHECK(!settings().scrollbar_show_delay.is_zero());
+      DCHECK(!settings().scrollbar_fade_in_duration.is_zero());
       base::TimeDelta show_delay = settings().scrollbar_show_delay;
+      base::TimeDelta fade_in_duration = settings().scrollbar_fade_in_duration;
       base::TimeDelta thinning_duration =
           settings().scrollbar_thinning_duration;
       return ScrollbarAnimationController::
           CreateScrollbarAnimationControllerAuraOverlay(
               scroll_layer_id, layer_tree_host_impl_, show_delay,
-              fade_out_delay, fade_out_resize_delay, fade_out_duration,
-              thinning_duration);
+              fade_out_delay, fade_out_resize_delay, fade_in_duration,
+              fade_out_duration, thinning_duration);
     }
     case LayerTreeSettings::NO_ANIMATOR:
       NOTREACHED();