Call setNeedsRedraw when scrollbar opacity changes.

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 da3ba8a2e204235b935a5fa88d0dc97c5c5ff269..576ec2a4318ce93d1955db96d7a206c844e5b54b 100644
--- a/cc/input/scrollbar_animation_controller.cc
+++ b/cc/input/scrollbar_animation_controller.cc
@@ -170,7 +170,6 @@ float ScrollbarAnimationController::AnimationProgressAtTime(
 
 void ScrollbarAnimationController::RunAnimationFrame(float progress) {
   ApplyOpacityToScrollbars(1.f - progress);
-  client_->SetNeedsRedrawForScrollbarAnimation();
   if (progress == 1.f)
     StopAnimation();
 }
@@ -377,6 +376,9 @@ void ScrollbarAnimationController::ApplyOpacityToScrollbars(float opacity) {
   bool previouslyVisible = opacity_ > 0.0f;
   bool currentlyVisible = opacity > 0.0f;
 
+  if (opacity_ != opacity)
+    client_->SetNeedsRedrawForScrollbarAnimation();
+
   opacity_ = opacity;
 
   if (previouslyVisible != currentlyVisible)