referrer_charset is a lie. It's really the user's default_charset.

net/base/net_util.h

Index: net/base/net_util.h
===================================================================
--- net/base/net_util.h	(revision 119865)
+++ net/base/net_util.h	(working copy)
@@ -134,7 +134,7 @@
 
 // TODO(abarth): Move these functions to http_content_disposition.cc.
 bool DecodeFilenameValue(const std::string& input,
-                         const std::string& referrer_charset,
+                         const std::string& default_charset,
                          std::string* output);
 bool DecodeExtValue(const std::string& value, std::string* output);
 
@@ -209,11 +209,11 @@
 // order):
 //
 // 1) The raw Content-Disposition header in |content_disposition| (as read from
-//    the network.  |referrer_charset| is used as described in the comment for
+//    the network.  |default_charset| is used as described in the comment for
 //    GetFileNameFromCD().
 // 2) |suggested_name| if specified.  |suggested_name| is assumed to be in
 //    UTF-8.
-// 3) The filename extracted from the |url|.  |referrer_charset| will be used to
+// 3) The filename extracted from the |url|.  |default_charset| will be used to
 //    interpret the URL if there are non-ascii characters.
 // 4) |default_name|.  If non-empty, |default_name| is assumed to be a filename
 //    and shouldn't contain a path.  |default_name| is not subject to validation
@@ -235,7 +235,7 @@
 // thread that allows IO.
 NET_EXPORT string16 GetSuggestedFilename(const GURL& url,
                                          const std::string& content_disposition,
-                                         const std::string& referrer_charset,
+                                         const std::string& default_charset,
                                          const std::string& suggested_name,
                                          const std::string& mime_type,
                                          const std::string& default_name);
@@ -243,7 +243,7 @@
 // Similar to GetSuggestedFilename(), but returns a FilePath.
 NET_EXPORT FilePath GenerateFileName(const GURL& url,
                                      const std::string& content_disposition,
-                                     const std::string& referrer_charset,
+                                     const std::string& default_charset,
                                      const std::string& suggested_name,
                                      const std::string& mime_type,
                                      const std::string& default_name);