Use overlay scrollbars by default in CrOS native UI.

BUG=672199

Review-Url: https://codereview.chromium.org/2558173006
Cr-Commit-Position: refs/heads/master@{#438730}

ash/common/system/tray/fixed_sized_scroll_view.cc

@@ -5,7 +5,6 @@
 #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 {
 
@@ -19,10 +18,6 @@ bool UseMd() {
 
 FixedSizedScrollView::FixedSizedScrollView() {
   set_notify_enter_exit_on_child(true);
-  if (UseMd()) {
-    SetVerticalScrollBar(new views::OverlayScrollBar(false));
-    SetHorizontalScrollBar(new views::OverlayScrollBar(true));
-  }
 }
 
 FixedSizedScrollView::~FixedSizedScrollView() {}

ui/views/style/platform_style.cc

@@ -17,7 +17,9 @@
 #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
 
@@ -56,7 +58,11 @@ gfx::ImageSkia PlatformStyle::CreateComboboxArrow(bool is_enabled,
 
 // 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