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 109 matching lines @@
                      scoped_refptr<base::RefCountedMemory>(
                          base::RefCountedString::TakeString(&empty)));
 }
 
 bool OSExchangeDataProviderAuraX11::DidOriginateFromRenderer() const {
   return format_map_.find(atom_cache_.GetAtom(kRendererTaint)) !=
          format_map_.end();
 }
 
 void OSExchangeDataProviderAuraX11::SetString(const base::string16& text_data) {
+  if (HasString())
+    return;
+
   std::string utf8 = base::UTF16ToUTF8(text_data);
   scoped_refptr<base::RefCountedMemory> mem(
       base::RefCountedString::TakeString(&utf8));
 
   format_map_.Insert(atom_cache_.GetAtom(Clipboard::kMimeTypeText), mem);
   format_map_.Insert(atom_cache_.GetAtom(kText), mem);
   format_map_.Insert(atom_cache_.GetAtom(kString), mem);
   format_map_.Insert(atom_cache_.GetAtom(kUtf8String), mem);
 }
 
@@ skipping 401 matching lines @@
 
 ///////////////////////////////////////////////////////////////////////////////
 // OSExchangeData, public:
 
 // static
 OSExchangeData::Provider* OSExchangeData::CreateProvider() {
   return new OSExchangeDataProviderAuraX11();
 }
 
 }  // namespace ui