Make a test not depend on system settings.

content/browser/theme_helper_mac.mm

Index: content/browser/theme_helper_mac.mm
diff --git a/content/browser/theme_helper_mac.mm b/content/browser/theme_helper_mac.mm
index aa5ebd05a0d2fadd8a334ee871dc5e5da691d395..149836316892babb621c1c6361a162300c15fa37 100644
--- a/content/browser/theme_helper_mac.mm
+++ b/content/browser/theme_helper_mac.mm
@@ -22,6 +22,9 @@ using content::ThemeHelperMac;
 
 namespace {
 
+content::ScopedPreferredScrollerStyleOveride*
+    g_preferred_scroller_style_override = nullptr;
+
 bool GetScrollAnimationEnabled() {
   bool enabled = false;
   id value = nil;
@@ -185,6 +188,18 @@ ViewMsg_SystemColorsChanged* CreateSystemColorsChangedMessage() {
 
 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,
@@ -193,6 +208,8 @@ ThemeHelperMac* ThemeHelperMac::GetInstance() {
 
 // 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]);