Reland Fade out overlay scrollbar after page load

cc/trees/layer_tree_impl.cc

 // Copyright 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/trees/layer_tree_impl.h"
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include <algorithm>
@@ skipping 1676 matching lines @@
 }
 
 void LayerTreeImpl::RegisterScrollLayer(LayerImpl* layer) {
   if (layer->scroll_clip_layer_id() == Layer::INVALID_ID)
     return;
 
   clip_scroll_map_.insert(
       std::pair<int, int>(layer->scroll_clip_layer_id(), layer->id()));
 
   DidUpdateScrollState(layer->id());
+
+  if (settings().scrollbar_animator == LayerTreeSettings::AURA_OVERLAY)
+    layer->set_needs_show_scrollbars(true);
 }
 
 void LayerTreeImpl::UnregisterScrollLayer(LayerImpl* layer) {
   if (layer->scroll_clip_layer_id() == Layer::INVALID_ID)
     return;
 
   clip_scroll_map_.erase(layer->scroll_clip_layer_id());
 }
 
 void LayerTreeImpl::AddSurfaceLayer(LayerImpl* layer) {
@@ skipping 376 matching lines @@
 
 void LayerTreeImpl::ResetAllChangeTracking() {
   layers_that_should_push_properties_.clear();
   // Iterate over all layers, including masks.
   for (auto& layer : *layers_)
     layer->ResetChangeTracking();
   property_trees_.ResetAllChangeTracking();
 }
 
 }  // namespace cc