Aura overlay scrollbar stroke should be only 3 sided.

ui/native_theme/native_theme_aura.cc

 // Copyright (c) 2012 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.
 
 #include "ui/native_theme/native_theme_aura.h"
 
 #include <limits>
 #include <utility>
 
 #include "base/logging.h"
@@ skipping 215 matching lines @@
         SkColorSetA(kOverlayScrollbarStrokeColor[theme], thumb_alpha));
     paint.setStyle(SkPaint::kStroke_Style);
     paint.setStrokeWidth(kStrokeWidth);
 
     gfx::RectF stroke_rect(thumb_rect);
     constexpr float kHalfStrokeWidth = kStrokeWidth / 2.f;
     stroke_rect.Inset(kHalfStrokeWidth, kHalfStrokeWidth);
     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.
     const int kThumbPadding = 2;
     const int extra_padding =
         (scrollbar_button_length() == 0) ? kThumbPadding : 0;
     if (part == NativeTheme::kScrollbarVerticalThumb)
       thumb_rect.Inset(kThumbPadding, extra_padding);
     else
       thumb_rect.Inset(extra_padding, kThumbPadding);
@@ skipping 35 matching lines @@
         // scrollbars isn't asking for part sizes that don't exist.
         // crbug.com/657159.
         break;
     }
   }
 
   return NativeThemeBase::GetPartSize(part, state, extra);
 }
 
 }  // namespace ui