Add flag to hide scrollbars completely.

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 873af518985c1dc6226256118424e2a6493dd086..fc8c0ff40285a2a6de36ce94df2f3e466b12bd40 100644
--- a/ui/native_theme/native_theme_switches.cc
+++ b/ui/native_theme/native_theme_switches.cc
@@ -13,6 +13,9 @@ 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 {
@@ -21,6 +24,10 @@ 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))
@@ -29,4 +36,10 @@ bool IsOverlayScrollbarEnabled() {
   return false;
 }
 
+bool ShouldHideScrollbars() {
+  const base::CommandLine& command_line =
+      *base::CommandLine::ForCurrentProcess();
+  return command_line.HasSwitch(switches::kHideScrollbars);
+}
+
 }  // namespace ui