Disable overlay scrollbars in Blink when hidden by the compositor.

cc/input/scrollbar_animation_controller_thinning.cc

Index: cc/input/scrollbar_animation_controller_thinning.cc
diff --git a/cc/input/scrollbar_animation_controller_thinning.cc b/cc/input/scrollbar_animation_controller_thinning.cc
index 5100988bc3299dfb3d4ae3bb600f1169d0c3059b..3da421778649aebf47f449d1dd684488b93f0231 100644
--- a/cc/input/scrollbar_animation_controller_thinning.cc
+++ b/cc/input/scrollbar_animation_controller_thinning.cc
@@ -214,6 +214,9 @@ void ScrollbarAnimationControllerThinning::ApplyOpacity(float opacity) {
   }
 
   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(