Make aura_demo work on Mac

ui/base/clipboard/custom_data_helper.h

 // Copyright (c) 2012 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.
 //
 // Due to restrictions of most operating systems, we don't directly map each
 // type of custom data to a native data transfer type. Instead, we serialize
 // each key-value pair into the pickle as a pair of string objects, and then
 // write the binary data in the pickle to the native data transfer object.
 
 #ifndef UI_BASE_CLIPBOARD_CUSTOM_DATA_HELPER_H_
@@ skipping 10 matching lines @@
 #if defined(OS_MACOSX)
 #ifdef __OBJC__
 @class NSString;
 #else
 class NSString;
 #endif
 #endif  // defined(OS_MACOSX)
 
 namespace ui {
 
-#if defined(OS_MACOSX)
+#if defined(OS_MACOSX) && !defined(USE_AURA)
 UI_BASE_EXPORT extern NSString* const kWebCustomDataPboardType;
 #elif (!defined(OS_WIN) && defined(USE_AURA)) || defined(TOOLKIT_GTK)
 UI_BASE_EXPORT extern const char kMimeTypeWebCustomData[];
 #endif
 
 UI_BASE_EXPORT void ReadCustomDataTypes(const void* data,
                                         size_t data_length,
                                         std::vector<base::string16>* types);
 UI_BASE_EXPORT void ReadCustomDataForType(const void* data,
                                           size_t data_length,
                                           const base::string16& type,
                                           base::string16* result);
 UI_BASE_EXPORT void ReadCustomDataIntoMap(
     const void* data,
     size_t data_length,
     std::map<base::string16, base::string16>* result);
 
 UI_BASE_EXPORT void WriteCustomDataToPickle(
     const std::map<base::string16, base::string16>& data,
     Pickle* pickle);
 
 }  // namespace ui
 
 #endif  // UI_BASE_CLIPBOARD_CLIPBOARD_H_