Show Overlay Scrollbar when GestureScrollUpdate

cc/trees/layer_tree_host_impl.cc

Index: cc/trees/layer_tree_host_impl.cc
diff --git a/cc/trees/layer_tree_host_impl.cc b/cc/trees/layer_tree_host_impl.cc
index 132616ef303232e01822a4a4dd03ebb7dd1dbb86..4cd466553ca261e4e3f51a51b753fb1955ed1fcc 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -2856,6 +2856,17 @@ InputHandler::ScrollStatus LayerTreeHostImpl::ScrollAnimated(
       MainThreadScrollingReason::kNotScrollingOnMain;
   ScrollTree& scroll_tree = active_tree_->property_trees()->scroll_tree;
   ScrollNode* scroll_node = scroll_tree.CurrentlyScrollingNode();
+
+  LayerImpl* currently_scrolling_layer = CurrentlyScrollingLayer();
+
+  if (currently_scrolling_layer) {
+    ScrollbarAnimationController* animation_controller =
+        ScrollbarAnimationControllerForId(currently_scrolling_layer->id());
+
+    if (animation_controller)
+      animation_controller->WillUpdateScroll();
+  }
+
   if (scroll_node) {
     gfx::Vector2dF delta = scroll_delta;
     if (!scroll_node->user_scrollable_horizontal)
@@ -3142,9 +3153,17 @@ InputHandlerScrollResult LayerTreeHostImpl::ScrollBy(
   DCHECK(scroll_state);
 
   TRACE_EVENT0("cc", "LayerTreeHostImpl::ScrollBy");
-  if (!CurrentlyScrollingLayer())
+
+  LayerImpl* currently_scrolling_layer = CurrentlyScrollingLayer();
+  if (!currently_scrolling_layer)
     return InputHandlerScrollResult();
 
+  ScrollbarAnimationController* animation_controller =
+      ScrollbarAnimationControllerForId(currently_scrolling_layer->id());
+
+  if (animation_controller)
+    animation_controller->WillUpdateScroll();
+
   float initial_top_controls_offset =
       browser_controls_offset_manager_->ControlsTopOffset();