Mac: Remove some remaining 10.6 logic for absent overlay scrollbar APIs

ui/base/test/scoped_preferred_scroller_style_mac.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/base/test/scoped_preferred_scroller_style_mac.h"
 
 #import "base/mac/sdk_forward_declarations.h"
 #import "base/mac/scoped_objc_class_swizzler.h"
 
 using base::mac::ScopedObjCClassSwizzler;
@@ skipping 36 matching lines @@
   return NSScrollerStyleOverlay;
 }
 
 @end
 
 namespace ui {
 namespace test {
 
 ScopedPreferredScrollerStyle::ScopedPreferredScrollerStyle(bool overlay)
     : overlay_(overlay) {
-  if (![NSScroller respondsToSelector:@selector(preferredScrollerStyle)])
-    return;
-
   NSInteger previous_style = [NSScroller preferredScrollerStyle];
   Class style_class = overlay_
                           ? [FakeNSScrollerPreferredStyleOverlayDonor class]
                           : [FakeNSScrollerPreferredStyleLegacyDonor class];
 
   swizzler_.reset(new ScopedObjCClassSwizzler(
       [NSScroller class], style_class, @selector(preferredScrollerStyle)));
 
   if (previous_style != GetScrollerStyle(overlay_))
     NotifyStyleChanged();
 }
 
 ScopedPreferredScrollerStyle::~ScopedPreferredScrollerStyle() {
-  if (!swizzler_)
-    return;  // Handle 10.6, which wouldn't have swizzled anything.
-
   swizzler_.reset();
 
   if ([NSScroller preferredScrollerStyle] != GetScrollerStyle(overlay_))
     NotifyStyleChanged();
 }
 
 }  // namespace test
 }  // namespace ui