Alpha for overlay scrollbars decreased when hovered.

ui/native_theme/native_theme_aura.cc

Index: ui/native_theme/native_theme_aura.cc
diff --git a/ui/native_theme/native_theme_aura.cc b/ui/native_theme/native_theme_aura.cc
index 4b6742f795fc1ee9fec0e0ce63f11da37d94c27e..b3e90e5f8c203af8ed31d3901402bfbff1ee1579 100644
--- a/ui/native_theme/native_theme_aura.cc
+++ b/ui/native_theme/native_theme_aura.cc
@@ -35,6 +35,7 @@ constexpr int kOverlayScrollbarMinimumLength = 12;
 constexpr SkAlpha kOverlayScrollbarAlphaNormal = 0x4D;
 constexpr SkAlpha kOverlayScrollbarAlphaHovered = 0x80;
 constexpr SkAlpha kOverlayScrollbarAlphaPressed = 0x80;
+constexpr SkAlpha kOverlayStrokeAlphaHovered = 0x58;
 
 // Indexed by ScrollbarOverlayColorTheme.
 constexpr SkColor kOverlayScrollbarThumbColor[] = {SK_ColorBLACK,
@@ -222,8 +223,12 @@ void NativeThemeAura::PaintScrollbarThumb(
     // In overlay mode, draw a stroke (border).
     constexpr int kStrokeWidth = kOverlayScrollbarStrokeWidth;
     SkPaint paint;
+
+    SkAlpha stroke_alpha = state == NativeTheme::kHovered
+                               ? kOverlayStrokeAlphaHovered
+                               : thumb_alpha;

[bokan, 2016/12/15 17:59:28]

for consistency, replace thumb_alpha here with a new kOverlayStrokeAlphaNormal

     paint.setColor(
-        SkColorSetA(kOverlayScrollbarStrokeColor[theme], thumb_alpha));
+        SkColorSetA(kOverlayScrollbarStrokeColor[theme], stroke_alpha));
     paint.setStyle(SkPaint::kStroke_Style);
     paint.setStrokeWidth(kStrokeWidth);