Enable overlay scrollbars on ChromeOS

ui/native_theme/native_theme_switches.cc

 // Copyright 2014 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 "base/command_line.h"
 #include "ui/native_theme/native_theme_switches.h"
 
 namespace switches {
 
 // Enables or disables overlay scrollbars in Blink (i.e. web content) on Aura
 // or Linux.  The status of native UI overlay scrollbars are determined in
 // PlatformStyle::CreateScrollBar. Does nothing on Mac.
 const char kEnableOverlayScrollbar[] = "enable-overlay-scrollbar";
 const char kDisableOverlayScrollbar[] = "disable-overlay-scrollbar";
 
 }  // namespace switches
 
 namespace ui {
 
 bool IsOverlayScrollbarEnabled() {
   const base::CommandLine& command_line =
       *base::CommandLine::ForCurrentProcess();
 
   if (command_line.HasSwitch(switches::kDisableOverlayScrollbar))
     return false;
 
+#if defined(OS_CHROMEOS)
+  return true;
+#else
   return command_line.HasSwitch(switches::kEnableOverlayScrollbar);
+#endif
 }
 
 }  // namespace ui