Add flag to hide scrollbars completely.

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 overlay scrollbars on Aura or Linux. Does nothing on Mac.
 const char kEnableOverlayScrollbar[] = "enable-overlay-scrollbar";
 
 // Disables overlay scrollbars on Aura or Linux. Does nothing on Mac.
 const char kDisableOverlayScrollbar[] = "disable-overlay-scrollbar";
 
+// Hides scrollbars on Aura, Linux, Android, ChromeOS. Does nothing on Mac.
+const char kHideScrollbars[] = "hide-scrollbars";
+
 }  // namespace switches
 
 namespace ui {
 
 bool IsOverlayScrollbarEnabled() {
   const base::CommandLine& command_line =
       *base::CommandLine::ForCurrentProcess();
 
+  // Hidden scrollbars are realized through never-showing overlay scrollbars.
+  if (ShouldHideScrollbars())
+    return true;
+
   if (command_line.HasSwitch(switches::kDisableOverlayScrollbar))
     return false;
   else if (command_line.HasSwitch(switches::kEnableOverlayScrollbar))
     return true;
 
   return false;
 }
 
+bool ShouldHideScrollbars() {
+  const base::CommandLine& command_line =
+      *base::CommandLine::ForCurrentProcess();
+  return command_line.HasSwitch(switches::kHideScrollbars);
+}
+
 }  // namespace ui