mac: Remove IsOSSnowLeopard().

ui/views/cocoa/views_scrollbar_bridge.mm

 // Copyright 2016 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.
 
 #import "ui/views/cocoa/views_scrollbar_bridge.h"
 
-#include "base/mac/mac_util.h"
 #import "base/mac/sdk_forward_declarations.h"
 
 @interface ViewsScrollbarBridge ()
 
 // Called when we receive a NSPreferredScrollerStyleDidChangeNotification.
 - (void)onScrollerStyleChanged:(NSNotification*)notification;
 
 @end
 
 @implementation ViewsScrollbarBridge
@@ skipping 19 matching lines @@
   delegate_ = nullptr;
   [[NSNotificationCenter defaultCenter] removeObserver:self];
 }
 
 - (void)onScrollerStyleChanged:(NSNotification*)notification {
   if (delegate_)
     delegate_->OnScrollerStyleChanged();
 }
 
 + (NSScrollerStyle)getPreferredScrollerStyle {
-  if (![NSScroller respondsToSelector:@selector(preferredScrollerStyle)]) {
-    DCHECK(base::mac::IsOSSnowLeopard());
-    return NSScrollerStyleLegacy;
-  }
   return [NSScroller preferredScrollerStyle];
 }
 
-@end
+@end