Aura overlay scrollbar stroke should be only 3 sided.

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..6793a19820468cd38aa9ab88355cf6fff79239e9 100644
--- a/ui/native_theme/native_theme_aura.cc
+++ b/ui/native_theme/native_theme_aura.cc
@@ -233,7 +233,12 @@ void NativeThemeAura::PaintScrollbarThumb(
     canvas->drawRect(gfx::RectFToSkRect(stroke_rect), paint);
 
     // Inset the all the edges edges so we fill-in the stroke below.
-    thumb_rect.Inset(kStrokeWidth, kStrokeWidth);
+    // TODO(bokan): Need to flip this for text direction of scroller.
+    if (part == NativeTheme::kScrollbarVerticalThumb)
+      thumb_rect.Inset(kStrokeWidth, kStrokeWidth, 0, kStrokeWidth);
+    else
+      thumb_rect.Inset(kStrokeWidth, kStrokeWidth, kStrokeWidth, 0);
+
   } else {
     // If there are no scrollbuttons then provide some padding so that the thumb
     // doesn't touch the top of the track.