Fix NSPasteboard leaks.

ui/base/clipboard/clipboard_util_mac.h

 // 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.
 
 #ifndef UI_BASE_CLIPBOARD_CLIPBOARD_UTIL_MAC_H_
 #define UI_BASE_CLIPBOARD_CLIPBOARD_UTIL_MAC_H_
 
 #import <AppKit/AppKit.h>
 
 #include "base/mac/scoped_nsobject.h"
+#include "base/memory/ref_counted.h"
 #include "ui/base/ui_base_export.h"
 
 namespace ui {
 
+class UI_BASE_EXPORT UniquePasteboard
+    : public base::RefCounted<UniquePasteboard> {
+ public:
+  UniquePasteboard();
+
+  NSPasteboard* get() { return pasteboard_; }
+
+ private:
+  friend class base::RefCounted<UniquePasteboard>;
+  ~UniquePasteboard();
+  base::scoped_nsobject<NSPasteboard> pasteboard_;
+};
+
 class UI_BASE_EXPORT ClipboardUtil {
  public:
   // Returns an NSPasteboardItem that represents the given |url|.
   // |url| must not be nil.
   // If |title| is nil, |url| is used in its place.
   static base::scoped_nsobject<NSPasteboardItem> PasteboardItemFromUrl(
       NSString* url,
       NSString* title);
 };
 }
 
 #endif  // UI_BASE_CLIPBOARD_CLIPBOARD_UTIL_MAC_H_