Use the user's preferred downloads directory for creating the initial download file.

content/browser/download/base_file.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.
 
 #ifndef CONTENT_BROWSER_DOWNLOAD_BASE_FILE_H_
 #define CONTENT_BROWSER_DOWNLOAD_BASE_FILE_H_
 
 #include <string>
 
 #include "base/file_path.h"
@@ skipping 22 matching lines @@
            const GURL& source_url,
            const GURL& referrer_url,
            int64 received_bytes,
            bool calculate_hash,
            const std::string& hash_state,
            const linked_ptr<net::FileStream>& file_stream,
            const net::BoundNetLog& bound_net_log);
   virtual ~BaseFile();
 
   // Returns net::OK on success, or a network error code on failure.
-  net::Error Initialize();
+  // |default_directory| specifies the directory to create the temporary file in
+  // if |full_path()| is empty. If |default_directory| and |full_path()| are
+  // empty, then a temporary file will be created in the default download
+  // location as determined by ContentBrowserClient.
+  net::Error Initialize(const FilePath& default_directory);
 
   // Write a new chunk of data to the file.
   // Returns net::OK on success (all bytes written to the file),
   // or a network error code on failure.
   net::Error AppendDataToFile(const char* data, size_t data_len);
 
   // Rename the download file.
   // Returns net::OK on success, or a network error code on failure.
   virtual net::Error Rename(const FilePath& full_path);
 
@@ skipping 81 matching lines @@
   // Indicates that this class no longer owns the associated file, and so
   // won't delete it on destruction.
   bool detached_;
 
   net::BoundNetLog bound_net_log_;
 
   DISALLOW_COPY_AND_ASSIGN(BaseFile);
 };
 
 #endif  // CONTENT_BROWSER_DOWNLOAD_BASE_FILE_H_