Make a test not depend on system settings.

content/browser/theme_helper_mac.mm

 // Copyright 2013 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 "content/browser/theme_helper_mac.h"
 
 #import <Cocoa/Cocoa.h>
 
 #include "base/command_line.h"
 #include "base/mac/mac_util.h"
 #include "base/mac/sdk_forward_declarations.h"
 #include "base/strings/sys_string_conversions.h"
 #include "content/common/view_messages.h"
 #include "content/public/browser/browser_thread.h"
 #include "content/public/browser/notification_service.h"
 #include "content/public/browser/notification_types.h"
 #include "content/public/browser/render_process_host.h"
 #include "content/public/common/content_switches.h"
 
 using content::RenderProcessHost;
 using content::ThemeHelperMac;
 
 namespace {
 
+content::ScopedPreferredScrollerStyleOveride*
+    g_preferred_scroller_style_override = nullptr;
+
 bool GetScrollAnimationEnabled() {
   bool enabled = false;
   id value = nil;
   if (base::mac::IsOSMountainLionOrLater()) {
     value = [[NSUserDefaults standardUserDefaults]
         objectForKey:@"NSScrollAnimationEnabled"];
   } else {
     value = [[NSUserDefaults standardUserDefaults]
         objectForKey:@"AppleScrollAnimationEnabled"];
   }
@@ skipping 143 matching lines @@
        !it.IsAtEnd();
        it.Advance()) {
     it.GetCurrentValue()->Send(new ViewMsg_UpdateScrollbarTheme(params));
   }
 }
 
 @end
 
 namespace content {
 
+ScopedPreferredScrollerStyleOveride::ScopedPreferredScrollerStyleOveride(
+    blink::ScrollerStyle value) : value_(value) {
+  DCHECK(!g_preferred_scroller_style_override);
+  g_preferred_scroller_style_override = this;
+  [ScrollbarPrefsObserver notifyPrefsChangedWithRedraw:NO];
+}
+
+ScopedPreferredScrollerStyleOveride::~ScopedPreferredScrollerStyleOveride() {
+  g_preferred_scroller_style_override = nullptr;
+  [ScrollbarPrefsObserver notifyPrefsChangedWithRedraw:NO];
+}
+
 // static
 ThemeHelperMac* ThemeHelperMac::GetInstance() {
   return base::Singleton<ThemeHelperMac,
                          base::LeakySingletonTraits<ThemeHelperMac>>::get();
 }
 
 // static
 blink::ScrollerStyle ThemeHelperMac::GetPreferredScrollerStyle() {
+  if (g_preferred_scroller_style_override)
+    return g_preferred_scroller_style_override->value();
   if (![NSScroller respondsToSelector:@selector(preferredScrollerStyle)])
     return blink::ScrollerStyleLegacy;
   return static_cast<blink::ScrollerStyle>([NSScroller preferredScrollerStyle]);
 }
 
 ThemeHelperMac::ThemeHelperMac() {
   [ScrollbarPrefsObserver registerAsObserver];
   registrar_.Add(this,
                  NOTIFICATION_RENDERER_PROCESS_CREATED,
                  NotificationService::AllSources());
@@ skipping 12 matching lines @@
   ViewMsg_UpdateScrollbarTheme_Params params;
   FillScrollbarThemeParams(&params);
   params.redraw = false;
 
   RenderProcessHost* rph = Source<RenderProcessHost>(source).ptr();
   rph->Send(new ViewMsg_UpdateScrollbarTheme(params));
   rph->Send(CreateSystemColorsChangedMessage());
 }
 
 }  // namespace content