Use overlay scrollbars by default in CrOS native UI.

ash/common/system/tray/fixed_sized_scroll_view.cc

 // Copyright 2013 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 "ash/common/system/tray/fixed_sized_scroll_view.h"
 
 #include "ash/common/material_design/material_design_controller.h"
-#include "ui/views/controls/scrollbar/overlay_scroll_bar.h"
 
 namespace ash {
 
 namespace {
 
 bool UseMd() {
   return MaterialDesignController::IsSystemTrayMenuMaterial();
 }
 
 }  // namespace
 
 FixedSizedScrollView::FixedSizedScrollView() {
   set_notify_enter_exit_on_child(true);
-  if (UseMd()) {
-    SetVerticalScrollBar(new views::OverlayScrollBar(false));
-    SetHorizontalScrollBar(new views::OverlayScrollBar(true));
-  }
 }
 
 FixedSizedScrollView::~FixedSizedScrollView() {}
 
 void FixedSizedScrollView::SetContentsView(views::View* view) {
   SetContents(view);
   if (!UseMd())
     view->SetBoundsRect(gfx::Rect(view->GetPreferredSize()));
 }
 
@@ skipping 45 matching lines @@
 void FixedSizedScrollView::OnBoundsChanged(const gfx::Rect& previous_bounds) {
   if (UseMd())
     return;
 
   gfx::Rect bounds = gfx::Rect(contents()->GetPreferredSize());
   bounds.set_width(std::max(0, width() - GetScrollBarLayoutWidth()));
   contents()->SetBoundsRect(bounds);
 }
 
 }  // namespace ash