Make Clipboard::FormatType an opaque handle type.

ui/base/clipboard/scoped_clipboard_writer.h

 // Copyright (c) 2011 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.
 
 // This file declares the ScopedClipboardWriter class, a wrapper around
 // the Clipboard class which simplifies writing data to the system clipboard.
 // Upon deletion the class atomically writes all data to |clipboard_|,
 // avoiding any potential race condition with other processes that are also
 // writing to the system clipboard.
 
@@ skipping 42 matching lines @@
   void WriteHyperlink(const string16& anchor_text, const std::string& url);
 
   // Used by WebKit to determine whether WebKit wrote the clipboard last
   void WriteWebSmartPaste();
 
   // Adds a bitmap to the clipboard
   // Pixel format is assumed to be 32-bit BI_RGB.
   void WriteBitmapFromPixels(const void* pixels, const gfx::Size& size);
 
   // Adds arbitrary data to clipboard.
-  void WritePickledData(const Pickle& pickle, Clipboard::FormatType format);
+  void WritePickledData(const Pickle& pickle,
+                        const Clipboard::FormatType& format);
 
  protected:
   // Converts |text| to UTF-8 and adds it to the clipboard.  If it's a URL, we
   // also notify the clipboard of that fact.
   void WriteTextOrURL(const string16& text, bool is_url);
 
   // We accumulate the data passed to the various targets in the |objects_|
   // vector, and pass it to Clipboard::WriteObjects() during object destruction.
   Clipboard::ObjectMap objects_;
   Clipboard* clipboard_;
 
   // We keep around the UTF-8 text of the URL in order to pass it to
   // Clipboard::DidWriteURL().
   std::string url_text_;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(ScopedClipboardWriter);
 };
 
 }  // namespace ui
 
 #endif  // UI_BASE_CLIPBOARD_SCOPED_CLIPBOARD_WRITER_H_