change overlay scrollbar hover show to hover fade in

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 1467 matching lines @@
   base::TimeDelta fade_out_duration = settings().scrollbar_fade_out_duration;
   switch (settings().scrollbar_animator) {
     case LayerTreeSettings::ANDROID_OVERLAY: {
       return ScrollbarAnimationController::
           CreateScrollbarAnimationControllerAndroid(
               scroll_layer_id, layer_tree_host_impl_, fade_out_delay,
               fade_out_resize_delay, fade_out_duration);
     }
     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();
       break;
   }
   return nullptr;
 }
 
 void LayerTreeImpl::DidAnimateScrollOffset() {
   layer_tree_host_impl_->DidAnimateScrollOffset();
@@ skipping 674 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