Reland of 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 d9f1a22aefd1c60effdaa1f527280a69a8ceda65..b6c6d60b199eaf97044f7771a007a20874c5a242 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -2904,7 +2904,15 @@ InputHandler::ScrollStatus LayerTreeHostImpl::ScrollAnimated(
       MainThreadScrollingReason::kNotScrollingOnMain;
   ScrollTree& scroll_tree = active_tree_->property_trees()->scroll_tree;
   ScrollNode* scroll_node = scroll_tree.CurrentlyScrollingNode();
+
   if (scroll_node) {
+    // Flash the overlay scrollbar even if the scroll dalta is 0.
+    ScrollbarAnimationController* animation_controller =
+        ScrollbarAnimationControllerForId(scroll_node->owning_layer_id);
+
+    if (animation_controller)
+      animation_controller->WillUpdateScroll();
+
     gfx::Vector2dF delta = scroll_delta;
     if (!scroll_node->user_scrollable_horizontal)
       delta.set_x(0);
@@ -2943,6 +2951,13 @@ InputHandler::ScrollStatus LayerTreeHostImpl::ScrollAnimated(
           viewport()->MainScrollLayer() &&
           viewport()->MainScrollLayer()->scroll_tree_index() == scroll_node->id;
       if (scrolls_main_viewport_scroll_layer) {
+        // Flash the overlay scrollbar even if the scroll dalta is 0.
+        ScrollbarAnimationController* animation_controller =
+            ScrollbarAnimationControllerForId(scroll_node->owning_layer_id);
+
+        if (animation_controller)
+          animation_controller->WillUpdateScroll();
+
         gfx::Vector2dF scrolled =
             viewport()->ScrollAnimated(pending_delta, delayed_by);
         // Viewport::ScrollAnimated returns pending_delta as long as it starts
@@ -3187,9 +3202,18 @@ InputHandlerScrollResult LayerTreeHostImpl::ScrollBy(
   DCHECK(scroll_state);
 
   TRACE_EVENT0("cc", "LayerTreeHostImpl::ScrollBy");
-  if (!CurrentlyScrollingNode())
+  ScrollTree& scroll_tree = active_tree_->property_trees()->scroll_tree;
+  ScrollNode* scroll_node = scroll_tree.CurrentlyScrollingNode();
+
+  if (!scroll_node)
     return InputHandlerScrollResult();
 
+  ScrollbarAnimationController* animation_controller =
+      ScrollbarAnimationControllerForId(scroll_node->owning_layer_id);
+
+  if (animation_controller)
+    animation_controller->WillUpdateScroll();
+
   float initial_top_controls_offset =
       browser_controls_offset_manager_->ControlsTopOffset();