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

content/browser/download/base_file.h

Index: content/browser/download/base_file.h
diff --git a/content/browser/download/base_file.h b/content/browser/download/base_file.h
index 7ada03c7c4c9a0e7ec016c775aeb48531f713bbf..0854f8cf8a537b783345922feb6307b62607534f 100644
--- a/content/browser/download/base_file.h
+++ b/content/browser/download/base_file.h
@@ -29,7 +29,11 @@ class FileStream;
 // for DownloadFile and SaveFile, which keep more state information.
 class CONTENT_EXPORT BaseFile {
  public:
-  BaseFile(const FilePath& full_path,
+  // Either |download_directory| or |full_path| should be non-empty. If
+  // |full_path| isn't specified, a temporary file will be created under
+  // |download_directory|.
+  BaseFile(const FilePath& download_directory,
+           const FilePath& full_path,
            const GURL& source_url,
            const GURL& referrer_url,
            int64 received_bytes,
@@ -108,6 +112,9 @@ class CONTENT_EXPORT BaseFile {
   static const size_t kSha256HashLen = 32;
   static const unsigned char kEmptySha256Hash[kSha256HashLen];
 
+  // Default download directory to use for temporary file
+  FilePath download_directory_;

[Randy Smith (Not in Mondays), 2012/09/14 16:38:16]

I'm tempted to suggest that instead of doing this we switch the full_path_ into
"directory_" and "name_", and create the temp file in directory_ if name_ is
null.  They're both ugly, but that seems a little more flexible.  Why should
BaseFile have knowledge of the download_directory_?

[asanka, 2012/09/14 19:11:45]

I added a parameter to Initialize() instead and called it default_directory.

+
   // Source URL for the file being downloaded.
   GURL source_url_;