Views: Give ScrollViewTest a test harness.

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 <AppKit/AppKit.h>
 
+#include "base/logging.h"
 #import "base/mac/scoped_objc_class_swizzler.h"
 
 using base::mac::ScopedObjCClassSwizzler;
 
 namespace {
 
+// Swizzling can be stacked, but not interleaved without creating unexpected
+// states. Require that there is only one swizzler rather than tracking a stack.
+bool g_swizzling = false;
+
 void NotifyStyleChanged() {
   [[NSNotificationCenter defaultCenter]
       postNotificationName:NSPreferredScrollerStyleDidChangeNotification
                     object:nil];
 }
 
 NSScrollerStyle GetScrollerStyle(bool overlay) {
   return overlay ? NSScrollerStyleOverlay : NSScrollerStyleLegacy;
 }
 
@@ skipping 28 matching lines @@
 namespace ui {
 namespace test {
 
 ScopedPreferredScrollerStyle::ScopedPreferredScrollerStyle(bool overlay)
     : overlay_(overlay) {
   NSInteger previous_style = [NSScroller preferredScrollerStyle];
   Class style_class = overlay_
                           ? [FakeNSScrollerPreferredStyleOverlayDonor class]
                           : [FakeNSScrollerPreferredStyleLegacyDonor class];
 
+  DCHECK(!g_swizzling);
+  g_swizzling = true;
   swizzler_.reset(new ScopedObjCClassSwizzler(
       [NSScroller class], style_class, @selector(preferredScrollerStyle)));
 
   if (previous_style != GetScrollerStyle(overlay_))
     NotifyStyleChanged();
 }
 
 ScopedPreferredScrollerStyle::~ScopedPreferredScrollerStyle() {
   swizzler_.reset();
+  DCHECK(g_swizzling);
+  g_swizzling = false;
 
   if ([NSScroller preferredScrollerStyle] != GetScrollerStyle(overlay_))
     NotifyStyleChanged();
 }
 
 }  // namespace test
 }  // namespace ui