Use overlay scrollbars by default in CrOS native UI.

ui/views/style/platform_style.cc

 // Copyright 2015 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/views/style/platform_style.h"
 
 #include "base/memory/ptr_util.h"
 #include "build/build_config.h"
 #include "ui/base/material_design/material_design_controller.h"
 #include "ui/base/resource/resource_bundle.h"
 #include "ui/gfx/shadow_value.h"
 #include "ui/native_theme/native_theme.h"
 #include "ui/resources/grit/ui_resources.h"
 #include "ui/views/background.h"
 #include "ui/views/controls/button/label_button.h"
 #include "ui/views/controls/button/label_button_border.h"
 #include "ui/views/controls/focusable_border.h"
 #include "ui/views/controls/scrollbar/scroll_bar_views.h"
 
-#if defined(OS_LINUX) && !defined(OS_CHROMEOS)
+#if defined(OS_CHROMEOS)
+#include "ui/views/controls/scrollbar/overlay_scroll_bar.h"
+#elif defined(OS_LINUX)
 #define DESKTOP_LINUX
 #endif
 
 namespace views {
 namespace {
 
 #if !defined(DESKTOP_LINUX) && !defined(OS_MACOSX)
 // Default text and shadow colors for STYLE_BUTTON.
 const SkColor kStyleButtonTextColor = SK_ColorBLACK;
 const SkColor kStyleButtonShadowColor = SK_ColorWHITE;
@@ skipping 18 matching lines @@
 
 // static
 gfx::ImageSkia PlatformStyle::CreateComboboxArrow(bool is_enabled,
                                                   Combobox::Style style) {
   ui::ResourceBundle& rb = ui::ResourceBundle::GetSharedInstance();
   return *rb.GetImageSkiaNamed(IDR_MENU_DROPARROW);
 }
 
 // static
 std::unique_ptr<ScrollBar> PlatformStyle::CreateScrollBar(bool is_horizontal) {
+#if defined(OS_CHROMEOS)
+  return base::MakeUnique<OverlayScrollBar>(is_horizontal);
+#else
   return base::MakeUnique<ScrollBarViews>(is_horizontal);
+#endif
 }
 
 // static
 SkColor PlatformStyle::TextColorForButton(
     const ButtonColorByState& color_by_state,
     const LabelButton& button) {
   return color_by_state[button.state()];
 }
 
 // static
@@ skipping 18 matching lines @@
 
 #if !defined(DESKTOP_LINUX)
 // static
 std::unique_ptr<Border> PlatformStyle::CreateThemedLabelButtonBorder(
     LabelButton* button) {
   return button->CreateDefaultBorder();
 }
 #endif
 
 }  // namespace views