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

net/base/net_util.cc

Index: net/base/net_util.cc
===================================================================
--- net/base/net_util.cc	(revision 119865)
+++ net/base/net_util.cc	(working copy)
@@ -239,7 +239,7 @@
 }
 
 bool DecodeWord(const std::string& encoded_word,
-                const std::string& referrer_charset,
+                const std::string& default_charset,
                 bool* is_rfc2047,
                 std::string* output) {
   *is_rfc2047 = false;
@@ -248,13 +248,13 @@
     return true;
 
   if (!IsStringASCII(encoded_word)) {
-    // Try UTF-8, referrer_charset and the native OS default charset in turn.
+    // Try UTF-8, default_charset and the native OS default charset in turn.
     if (IsStringUTF8(encoded_word)) {
       *output = encoded_word;
     } else {
       string16 utf16_output;
-      if (!referrer_charset.empty() &&
-          base::CodepageToUTF16(encoded_word, referrer_charset.c_str(),
+      if (!default_charset.empty() &&
+          base::CodepageToUTF16(encoded_word, default_charset.c_str(),
                                 base::OnStringConversionError::FAIL,
                                 &utf16_output)) {
         *output = UTF16ToUTF8(utf16_output);
@@ -926,7 +926,7 @@
 // |should_overwrite_extension| will be set to true, in which case a better
 // extension should be determined based on the content type.
 std::string GetFileNameFromURL(const GURL& url,
-                               const std::string& referrer_charset,
+                               const std::string& default_charset,
                                bool* should_overwrite_extension) {
   // about: and data: URLs don't have file names, but esp. data: URLs may
   // contain parts that look like ones (i.e., contain a slash).  Therefore we
@@ -944,7 +944,7 @@
     bool ignore;
     // TODO(jshin): this is probably not robust enough. To be sure, we need
     // encoding detection.
-    DecodeWord(unescaped_url_filename, referrer_charset, &ignore,
+    DecodeWord(unescaped_url_filename, default_charset, &ignore,
                &decoded_filename);
   }
   // If the URL contains a (possibly empty) query, assume it is a generator, and
@@ -1187,7 +1187,7 @@
 }
 
 bool DecodeFilenameValue(const std::string& input,
-                         const std::string& referrer_charset,
+                         const std::string& default_charset,
                          std::string* output) {
   std::string tmp;
   // Tokenize with whitespace characters.
@@ -1209,7 +1209,7 @@
     // in a single encoded-word. Firefox/Thunderbird do not support
     // it, either.
     std::string decoded;
-    if (!DecodeWord(t.token(), referrer_charset, &is_previous_token_rfc2047,
+    if (!DecodeWord(t.token(), default_charset, &is_previous_token_rfc2047,
                     &decoded))
       return false;
     tmp.append(decoded);
@@ -1401,7 +1401,7 @@
 
 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) {
@@ -1416,7 +1416,7 @@
 
   // Try to extract a filename from content-disposition first.
   if (!content_disposition.empty()) {
-    HttpContentDisposition header(content_disposition, referrer_charset);
+    HttpContentDisposition header(content_disposition, default_charset);
     filename = header.filename();
   }
 
@@ -1428,7 +1428,7 @@
   // looks at the last component of the URL and doesn't return the hostname as a
   // failover.
   if (filename.empty())
-    filename = GetFileNameFromURL(url, referrer_charset, &overwrite_extension);
+    filename = GetFileNameFromURL(url, default_charset, &overwrite_extension);
 
   // Finally try the URL hostname, but only if there's no default specified in
   // |default_name|.  Some schemes (e.g.: file:, about:, data:) do not have a
@@ -1480,13 +1480,13 @@
 
 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_file_name) {
   string16 file_name = GetSuggestedFilename(url,
                                             content_disposition,
-                                            referrer_charset,
+                                            default_charset,
                                             suggested_name,
                                             mime_type,
                                             default_file_name);