Disable overlay scrollbars in Blink when hidden by the compositor.

cc/input/scrollbar_animation_controller.cc

Index: cc/input/scrollbar_animation_controller.cc
diff --git a/cc/input/scrollbar_animation_controller.cc b/cc/input/scrollbar_animation_controller.cc
index 086f63e30f939ef11c662d500b9bf51c2f3899a0..a8fb1d04d02b294432ff22c206bc8d3973042b94 100644
--- a/cc/input/scrollbar_animation_controller.cc
+++ b/cc/input/scrollbar_animation_controller.cc
@@ -42,6 +42,10 @@ bool ScrollbarAnimationController::Animate(base::TimeTicks now) {
   return true;
 }
 
+bool ScrollbarAnimationController::ScrollbarsHidden() const {
+  return false;
+}
+
 float ScrollbarAnimationController::AnimationProgressAtTime(
     base::TimeTicks now) {
   base::TimeDelta delta = now - last_awaken_time_;