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 65537995792dd13f5387a16e2a9c6a92f7ef2926..0a39e9d97d16febba841794063a1606ebce6181b 100644
--- a/ui/native_theme/overlay_scrollbar_constants_aura.h
+++ b/ui/native_theme/overlay_scrollbar_constants_aura.h
@@ -16,10 +16,12 @@ constexpr int kOverlayScrollbarThumbWidthPressed = 10;
 
 // kOverlayScrollbarShowDelay is only for the case where the mouse hovers near
 // the screen edge.
-constexpr base::TimeDelta kOverlayScrollbarShowDelay =
+constexpr base::TimeDelta kOverlayScrollbarFadeInDelay =
     base::TimeDelta::FromMilliseconds(1000);
 constexpr base::TimeDelta kOverlayScrollbarFadeOutDelay =
     base::TimeDelta::FromMilliseconds(1000);
+constexpr base::TimeDelta kOverlayScrollbarFadeInDuration =
+    base::TimeDelta::FromMilliseconds(200);
 constexpr base::TimeDelta kOverlayScrollbarFadeOutDuration =

[Evan Stade, 2017/04/19 22:05:32]

why do fade in/out have their own constants if they're identical?

     base::TimeDelta::FromMilliseconds(200);
 // TODO(bokan): This is still undetermined. crbug.com/652520.