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 kOverlayScrollbarStrokeWidth = 1;
 constexpr int kOverlayScrollbarThumbWidthPressed = 10;
 
 constexpr float kOverlayScrollbarThumbNormalAlpha = 0.5f;
 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 =
     base::TimeDelta::FromMilliseconds(200);
 
 }  // namespace ui
 
 #endif  // UI_NATIVE_THEME_OVERLAY_SCROLLBAR_CONSTANTS_AURA_H_