Disable overlay scrollbars in Blink when hidden by the compositor.

cc/input/scrollbar_animation_controller.cc

 // Copyright 2014 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.h"
 
 #include <algorithm>
 
 #include "base/time/time.h"
 #include "cc/trees/layer_tree_impl.h"
@@ skipping 24 matching lines @@
     last_awaken_time_ = now;
 
   float progress = AnimationProgressAtTime(now);
   RunAnimationFrame(progress);
 
   if (is_animating_)
     client_->SetNeedsAnimateForScrollbarAnimation();
   return true;
 }
 
+bool ScrollbarAnimationController::ScrollbarsHidden() const {
+  return false;
+}
+
 float ScrollbarAnimationController::AnimationProgressAtTime(
     base::TimeTicks now) {
   base::TimeDelta delta = now - last_awaken_time_;
   float progress = delta.InSecondsF() / Duration().InSecondsF();
   return std::max(std::min(progress, 1.f), 0.f);
 }
 
 void ScrollbarAnimationController::DidScrollBegin() {
   currently_scrolling_ = true;
 }
@@ skipping 38 matching lines @@
 void ScrollbarAnimationController::StopAnimation() {
   delayed_scrollbar_fade_.Cancel();
   is_animating_ = false;
 }
 
 ScrollbarSet ScrollbarAnimationController::Scrollbars() const {
   return client_->ScrollbarsFor(scroll_layer_id_);
 }
 
 }  // namespace cc