Enable overlay scrollbars on ChromeOS

ui/native_theme/native_theme_switches.cc

Index: ui/native_theme/native_theme_switches.cc
diff --git a/ui/native_theme/native_theme_switches.cc b/ui/native_theme/native_theme_switches.cc
index 9442e6d8483b024a57c642e4cbd6b7b4b4a622ba..1d8f8b3bf60d3c07b4633957617e3e47d7d3c8c1 100644
--- a/ui/native_theme/native_theme_switches.cc
+++ b/ui/native_theme/native_theme_switches.cc
@@ -24,7 +24,11 @@ bool IsOverlayScrollbarEnabled() {
   if (command_line.HasSwitch(switches::kDisableOverlayScrollbar))
     return false;
 
+#if defined(OS_CHROMEOS)
+  return true;
+#else
   return command_line.HasSwitch(switches::kEnableOverlayScrollbar);
+#endif
 }
 
 }  // namespace ui