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..0ba92055378ce085dfa53e95f0ffd280c3177e85 100644
--- a/ui/native_theme/overlay_scrollbar_constants_aura.h
+++ b/ui/native_theme/overlay_scrollbar_constants_aura.h
@@ -20,11 +20,11 @@ constexpr float kOverlayScrollbarStrokeHoverAlpha = 0.5f;
 
 // 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);

[Evan Stade, 2017/04/20 02:40:30]

Any reason to keep the fadein/out separate?

[chaopeng, 2017/04/20 02:50:10]

These two are different. FadeIn is by hovering a rect near screen edge. FadeOut
is by no scroll action. I think PM may change them to different value.

[Evan Stade, 2017/04/20 03:45:01]

I would say we should use the same constant (and in other code, the same
variable). If we need them to differ it won't be hard to allow for that.

 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 =