change overlay scrollbar hover show to hover fade in

ui/native_theme/overlay_scrollbar_constants_aura.h

Index: ui/native_theme/overlay_scrollbar_constants_aura.h
diff --git a/ui/native_theme/overlay_scrollbar_constants_aura.h b/ui/native_theme/overlay_scrollbar_constants_aura.h
index a6192d902249f49721dd6162b35e642024bc9f1e..feb344d704cc7136d76b009a15aca08d4fa9852b 100644
--- a/ui/native_theme/overlay_scrollbar_constants_aura.h
+++ b/ui/native_theme/overlay_scrollbar_constants_aura.h
@@ -18,13 +18,11 @@ constexpr float kOverlayScrollbarThumbHoverAlpha = 0.7f;
 constexpr float kOverlayScrollbarStrokeNormalAlpha = 0.3f;
 constexpr float kOverlayScrollbarStrokeHoverAlpha = 0.5f;
 
-// kOverlayScrollbarShowDelay is only for the case where the mouse hovers near
-// the screen edge.
-constexpr base::TimeDelta kOverlayScrollbarShowDelay =
+// kOverlayScrollbarFadeDelay is for the case where the mouse hovers near
+// the screen edge and no scroll action for a while.

[Evan Stade, 2017/04/20 14:33:45]

nit: the wording of this comment is a little confusing but I think the variable
names are self-documenting enough so instead of rewording it we can just delete
it.

+constexpr base::TimeDelta kOverlayScrollbarFadeDelay =
     base::TimeDelta::FromMilliseconds(1000);
-constexpr base::TimeDelta kOverlayScrollbarFadeOutDelay =
-    base::TimeDelta::FromMilliseconds(1000);
-constexpr base::TimeDelta kOverlayScrollbarFadeOutDuration =
+constexpr base::TimeDelta kOverlayScrollbarFadeDuration =
     base::TimeDelta::FromMilliseconds(200);
 // TODO(bokan): This is still undetermined. crbug.com/652520.
 constexpr base::TimeDelta kOverlayScrollbarThinningDuration =