change overlay scrollbar hover show to hover fade in

ui/native_theme/overlay_scrollbar_constants_aura.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef UI_NATIVE_THEME_OVERLAY_SCROLLBAR_CONSTANTS_AURA_H_
 #define UI_NATIVE_THEME_OVERLAY_SCROLLBAR_CONSTANTS_AURA_H_
 
 #include "base/time/time.h"
 #include "ui/gfx/skia_util.h"
 
 namespace ui {
 
 constexpr int kOverlayScrollbarThumbWidthNormal = 6;
 constexpr int kOverlayScrollbarThumbWidthHovered = 10;
 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.
 constexpr base::TimeDelta kOverlayScrollbarThinningDuration =
     base::TimeDelta::FromMilliseconds(200);
 
 }  // namespace ui
 
 #endif  // UI_NATIVE_THEME_OVERLAY_SCROLLBAR_CONSTANTS_AURA_H_