Overlay scrollbars are painted onload.

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 af02ad3cddb6d2ebf5b31d877bcecdba8abd8ac1..afc10f8ae25453048e517d47d4100eb94c13f5b0 100644
--- a/cc/input/scrollbar_animation_controller_thinning.cc
+++ b/cc/input/scrollbar_animation_controller_thinning.cc
@@ -124,12 +124,21 @@ float ScrollbarAnimationControllerThinning::
 float ScrollbarAnimationControllerThinning::AdjustScale(
     float new_value,
     float current_value,
-    AnimationChange animation_change) {
+    AnimationChange animation_change,
+    float min_value,
+    float max_value) {
+  float result;
   if (animation_change == INCREASE && current_value > new_value)
-    return current_value;
-  if (animation_change == DECREASE && current_value < new_value)
-    return current_value;
-  return new_value;
+    result = current_value;
+  else if (animation_change == DECREASE && current_value < new_value)
+    result = current_value;
+  else
+    result = new_value;
+  if (result > max_value)
+    return max_value;
+  if (result < min_value)
+    return min_value;
+  return result;
 }
 
 void ScrollbarAnimationControllerThinning::ApplyOpacityAndThumbThicknessScale(
@@ -140,9 +149,9 @@ void ScrollbarAnimationControllerThinning::ApplyOpacityAndThumbThicknessScale(
       continue;
     float effective_opacity =
         scrollbar->CanScrollOrientation()
-            ? AdjustScale(opacity, scrollbar->Opacity(), opacity_change_)
+            ? AdjustScale(opacity, scrollbar->Opacity(), opacity_change_,
+                          kIdleOpacity, 1)
             : 0;
-
     PropertyTrees* property_trees =
         scrollbar->layer_tree_impl()->property_trees();
     // If this method is called during LayerImpl::PushPropertiesTo, we may not
@@ -159,7 +168,7 @@ void ScrollbarAnimationControllerThinning::ApplyOpacityAndThumbThicknessScale(
     }
     scrollbar->SetThumbThicknessScaleFactor(AdjustScale(
         thumb_thickness_scale, scrollbar->thumb_thickness_scale_factor(),
-        thickness_change_));
+        thickness_change_, kIdleThicknessScale, 1));
   }
 }