Fix the HTML5 drag and drop demos on Linux and CrOS

ui/base/dragdrop/os_exchange_data_provider_aurax11.cc

 // 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.
 
 #include "ui/base/dragdrop/os_exchange_data_provider_aurax11.h"
 
 #include "base/logging.h"
 #include "base/memory/ref_counted_memory.h"
 #include "base/strings/string_util.h"
 #include "base/strings/utf_string_conversions.h"
@@ skipping 137 matching lines @@
     std::vector<unsigned char> data;
     ui::AddString16ToVector(spec, &data);
     ui::AddString16ToVector(base::ASCIIToUTF16("\n"), &data);
     ui::AddString16ToVector(title, &data);
     scoped_refptr<base::RefCountedMemory> mem(
         base::RefCountedBytes::TakeVector(&data));
 
     format_map_.Insert(atom_cache_.GetAtom(kMimeTypeMozillaURL), mem);
 
     // Set a string fallback as well.
-    SetString(spec);
+    if (!HasString())
+      SetString(spec);
 
     // Return early if this drag already contains file contents (this implies
     // that file contents must be populated before URLs). Nautilus (and possibly
     // other file managers) prefer _NETSCAPE_URL over the X Direct Save
     // protocol, but we want to prioritize XDS in this case.
     if (!file_contents_name_.empty())
       return;
 
     // Set _NETSCAPE_URL for file managers like Nautilus that use it as a hint
     // to create a link to the URL. Setting text/uri-list doesn't work because
@@ skipping 372 matching lines @@
 
 ///////////////////////////////////////////////////////////////////////////////
 // OSExchangeData, public:
 
 // static
 OSExchangeData::Provider* OSExchangeData::CreateProvider() {
   return new OSExchangeDataProviderAuraX11();
 }
 
 }  // namespace ui