Disable overlay scrollbars in Blink when hidden by the compositor.

cc/input/scrollbar_animation_controller_thinning.cc

 // Copyright 2013 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/input/scrollbar_animation_controller_thinning.h"
 
 #include "base/memory/ptr_util.h"
 #include "base/time/time.h"
 #include "cc/layers/layer_impl.h"
 #include "cc/layers/scrollbar_layer_impl_base.h"
@@ skipping 196 matching lines @@
     if (property_trees->IsInIdToIndexMap(PropertyTrees::TreeType::EFFECT,
                                          scrollbar->id())) {
       property_trees->effect_tree.OnOpacityAnimated(
           effective_opacity,
           property_trees->effect_id_to_index_map[scrollbar->id()],
           scrollbar->layer_tree_impl());
     }
   }
 
   opacity_ = opacity;
+
+  if (opacity_ == 0.0f || opacity_ == 1.0f)

[aelias_OOO_until_Jul13, 2016/10/26 19:01:03]

I think this if statement could just be removed with no ill effect, an
early-return in SetScrollbarsHidden should do the job and there's no reason to
treat 0.1 differently from 1.0.

[bokan, 2016/10/27 20:56:46]

Done.

+    client_->SetScrollbarsHidden(scroll_layer_id_, opacity_ == 0.0f);
 }
 
 void ScrollbarAnimationControllerThinning::ApplyThumbThicknessScale(
     float thumb_thickness_scale) {
   for (ScrollbarLayerImplBase* scrollbar : Scrollbars()) {
     if (!scrollbar->is_overlay_scrollbar())
       continue;
 
     scrollbar->SetThumbThicknessScaleFactor(AdjustScale(
         thumb_thickness_scale, scrollbar->thumb_thickness_scale_factor(),
         thickness_change_, kIdleThicknessScale, 1));
   }
 }
 
 void ScrollbarAnimationControllerThinning::SetCurrentAnimatingProperty(
     AnimatingProperty property) {
   if (current_animating_property_ == property)
     return;
 
   StopAnimation();
   current_animating_property_ = property;
   if (current_animating_property_ == THICKNESS)
     ApplyOpacity(1.f);
 }
 
 }  // namespace cc