Disable overlay scrollbars in Blink when hidden by the compositor.

cc/input/scrollbar_animation_controller_thinning_unittest.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 "cc/layers/solid_color_scrollbar_layer_impl.h"
 #include "cc/test/fake_impl_task_runner_provider.h"
 #include "cc/test/fake_layer_tree_host_impl.h"
 #include "cc/test/geometry_test_utils.h"
@@ skipping 26 matching lines @@
   }
   void SetNeedsRedrawForScrollbarAnimation() override {
     did_request_redraw_ = true;
   }
   void SetNeedsAnimateForScrollbarAnimation() override {
     did_request_animate_ = true;
   }
   ScrollbarSet ScrollbarsFor(int scroll_layer_id) const override {
     return host_impl_.ScrollbarsFor(scroll_layer_id);
   }
+  void SetScrollbarsHidden(int layer_id, bool hidden) override {}
 
  protected:
   const base::TimeDelta kDelayBeforeStarting = base::TimeDelta::FromSeconds(2);
   const base::TimeDelta kResizeDelayBeforeStarting =
       base::TimeDelta::FromSeconds(5);
   const base::TimeDelta kFadeDuration = base::TimeDelta::FromSeconds(3);
   const base::TimeDelta kThinningDuration = base::TimeDelta::FromSeconds(2);
 
   void SetUp() override {
     std::unique_ptr<LayerImpl> scroll_layer =
@@ skipping 869 matching lines @@
   scrollbar_controller_->Animate(time);
   EXPECT_FLOAT_EQ(0.5f, scrollbar_layer_->Opacity());
 
   time += kFadeDuration / 2;
   scrollbar_controller_->Animate(time);
   EXPECT_FLOAT_EQ(0.0f, scrollbar_layer_->Opacity());
 }
 
 }  // namespace
 }  // namespace cc