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..2006d98fddb44a0779c850a7be0c32edb12c5d1c 100644
--- a/cc/input/scrollbar_animation_controller_thinning.cc
+++ b/cc/input/scrollbar_animation_controller_thinning.cc
@@ -164,6 +164,10 @@ void ScrollbarAnimationControllerThinning::DidMouseMoveNear(float distance) {
   StartAnimation();
 }
 
+bool ScrollbarAnimationControllerThinning::ScrollbarsHidden() const {
+  return opacity_ == 0.0f;
+}
+
 float ScrollbarAnimationControllerThinning::ThumbThicknessScaleAt(
     float progress) {
   if (thickness_change_ == NONE)
@@ -213,7 +217,13 @@ void ScrollbarAnimationControllerThinning::ApplyOpacity(float opacity) {
     }
   }
 
+  bool previouslyVisible = opacity_ > 0.0f;
+  bool currentlyVisible = opacity > 0.0f;
+
   opacity_ = opacity;
+
+  if (previouslyVisible != currentlyVisible)
+    client_->DidChangeScrollbarVisibility();
 }
 
 void ScrollbarAnimationControllerThinning::ApplyThumbThicknessScale(