Fix FrameOwnerPropertiesPropagationScrolling on 10.6 after r372906

ui/base/test/scoped_preferred_scroller_style_legacy_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_legacy_mac.h"
 
 #import "base/mac/sdk_forward_declarations.h"
 #import "base/mac/scoped_objc_class_swizzler.h"
 
 using base::mac::ScopedObjCClassSwizzler;
@@ skipping 32 matching lines @@
   swizzler_.reset(
       new ScopedObjCClassSwizzler([NSScroller class],
                                   [FakeNSScrollerPreferredStyleDonor class],
                                   @selector(preferredScrollerStyle)));
 
   if (previous_style != NSScrollerStyleLegacy)
     NotifyStyleChanged();
 }
 
 ScopedPreferredScrollerStyleLegacy::~ScopedPreferredScrollerStyleLegacy() {
+  if (!swizzler_)
+    return;  // Handle 10.6, which wouldn't have swizzled anything.
+
   swizzler_.reset();
 
   if ([NSScroller preferredScrollerStyle] != NSScrollerStyleLegacy)
     NotifyStyleChanged();
 }
 
 }  // namespace test
 }  // namespace ui