Implement Clipboard::ReadData and Clipboard::WriteData for Mac.

ui/base/clipboard/clipboard_gtk.cc

 // 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.
 
 #include "ui/base/clipboard/clipboard.h"
 
 #include <gtk/gtk.h>
 #include <X11/extensions/Xfixes.h>
 #include <X11/Xatom.h>
 #include <map>
@@ skipping 482 matching lines @@
                                string16* result) const {
   // TODO(dcheng): Implement this.
   NOTIMPLEMENTED();
 }
 
 void Clipboard::ReadBookmark(string16* title, std::string* url) const {
   // TODO(estade): implement this.
   NOTIMPLEMENTED();
 }
 
-void Clipboard::ReadData(const std::string& format, std::string* result) {
+void Clipboard::ReadData(const std::string& format, std::string* result) const {
   GtkSelectionData* data =
       gtk_clipboard_wait_for_contents(clipboard_, StringToGdkAtom(format));
   if (!data)
     return;
   result->assign(reinterpret_cast<char*>(data->data), data->length);
   gtk_selection_data_free(data);
 }
 
 uint64 Clipboard::GetSequenceNumber(Buffer buffer) {
   if (buffer == BUFFER_STANDARD)
@@ skipping 40 matching lines @@
       return clipboard_;
     case BUFFER_SELECTION:
       return primary_selection_;
     default:
       NOTREACHED();
       return NULL;
   }
 }
 
 }  // namespace ui