Fix the HTML5 drag and drop demos on Linux and CrOS

ui/base/dragdrop/os_exchange_data_provider_aura.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_aura.h"
 
 #include "base/logging.h"
 #include "base/strings/utf_string_conversions.h"
 #include "net/base/net_util.h"
 #include "ui/base/clipboard/clipboard.h"
@@ skipping 26 matching lines @@
 void OSExchangeDataProviderAura::MarkOriginatedFromRenderer() {
   // TODO(dcheng): Currently unneeded because ChromeOS Aura correctly separates
   // URL and filename metadata, and does not implement the DownloadURL protocol.
 }
 
 bool OSExchangeDataProviderAura::DidOriginateFromRenderer() const {
   return false;
 }
 
 void OSExchangeDataProviderAura::SetString(const base::string16& data) {
+  if (HasString())
+    return;
+
   string_ = data;
   formats_ |= OSExchangeData::STRING;
 }
 
 void OSExchangeDataProviderAura::SetURL(const GURL& url,
                                         const base::string16& title) {
   url_ = url;
   title_ = title;
   formats_ |= OSExchangeData::URL;
 
@@ skipping 145 matching lines @@
 
 ///////////////////////////////////////////////////////////////////////////////
 // OSExchangeData, public:
 
 // static
 OSExchangeData::Provider* OSExchangeData::CreateProvider() {
   return new OSExchangeDataProviderAura();
 }
 
 }  // namespace ui