Make a test not depend on system settings.

content/browser/theme_helper_mac.h

 // 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.
 
 #ifndef CONTENT_BROWSER_THEME_HELPER_MAC_H_
 #define CONTENT_BROWSER_THEME_HELPER_MAC_H_
 
 #include "base/macros.h"
 #include "base/memory/singleton.h"
 #include "content/public/browser/notification_observer.h"
 #include "content/public/browser/notification_registrar.h"
 #include "third_party/WebKit/public/web/mac/WebScrollbarTheme.h"
 
 namespace content {
 
+class CONTENT_EXPORT ScopedPreferredScrollerStyleOveride {

[tapted, 2016/01/31 23:33:13]

ObjectiveC gives us a cheaty way to do this kind of stuff without affecting
production code, which is what we typicallly do. It's also likely we'll need
this elsewhere in ui, so I think it should go in

ui/base/test/scoped_preferred_scroller_style_mac.h

note it shouldn't/can't then have a blink:: dependency there, but that won't
matter.

See ui/base/test/scoepd_fake_nswindow_focus.h for something to cargo cult.
You'll end up with something like this (in the .mm)

...

#include ..sdk_forward_declarations.h

// Donates a testing implementation of +[NSScroller preferredScrollerStyle].
@interface FakeNSScrollerStyleDonor : NSObject
@end

@implementation FakeNSWindowFocusDonor

+ (NSScrollerStyle)preferredScrollerStyle {
  return NSScrollerStyleLegacy;
}

@end

namespace .... {

ScopedPreferredScrollerStyle::ScopedPreferredScrollerStyle() {
  if (![NSScroller respondsToSelector:@selector(preferredScrollerStyle)])
    return;
 
    swizzler_.reset(
          new ScopedObjCClassSwizzler([NSScroller class],
                                      [FakeNSScrollerStyleDonor class],
                                      @selector(preferredScrollerStyle)));

  /* use NSNotificaitonCenter to notify ScrollbarPrefsObserver */

}

+ public:
+  ScopedPreferredScrollerStyleOveride(blink::ScrollerStyle value);

[tapted, 2016/01/31 23:33:13]

nit: explicit

+  ~ScopedPreferredScrollerStyleOveride();
+
+  blink::ScrollerStyle value() const { return value_; }
+
+ private:
+  blink::ScrollerStyle value_;
+};

[tapted, 2016/01/31 23:33:13]

nit: DISALLOW_COPY_AND_ASSIGN(..)

+
 class ThemeHelperMac : public NotificationObserver {
  public:
   // Return pointer to the singleton instance for the current process, or NULL
   // if none.
   static ThemeHelperMac* GetInstance();
 
   // Returns the value of +[NSScroller preferredScrollStyle] as expressed
   // as the blink enum value.
   static blink::ScrollerStyle GetPreferredScrollerStyle();
 
  private:
   friend struct base::DefaultSingletonTraits<ThemeHelperMac>;
 
   ThemeHelperMac();
   ~ThemeHelperMac() override;
 
   // Overridden from NotificationObserver:
   void Observe(int type,
                const NotificationSource& source,
                const NotificationDetails& details) override;
 
   NotificationRegistrar registrar_;
 
   DISALLOW_COPY_AND_ASSIGN(ThemeHelperMac);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_BROWSER_THEME_HELPER_MAC_H_