Make a test not depend on system settings.

content/browser/theme_helper_mac.h

Index: content/browser/theme_helper_mac.h
diff --git a/content/browser/theme_helper_mac.h b/content/browser/theme_helper_mac.h
index be507d9535116cd0a5edbf81560ecc6bbc436175..a1aabfc4d87478b43fe52ce1722cee86ae47a96c 100644
--- a/content/browser/theme_helper_mac.h
+++ b/content/browser/theme_helper_mac.h
@@ -13,6 +13,17 @@
 
 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