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

net/base/net_util_unittest.cc

Index: net/base/net_util_unittest.cc
===================================================================
--- net/base/net_util_unittest.cc	(revision 119865)
+++ net/base/net_util_unittest.cc	(working copy)
@@ -45,7 +45,7 @@
 
 struct FileNameCDCase {
   const char* header_field;
-  const char* referrer_charset;
+  const char* default_charset;
   const wchar_t* expected;
 };
 
@@ -387,7 +387,7 @@
 struct GenerateFilenameCase {
   const char* url;
   const char* content_disp_header;
-  const char* referrer_charset;
+  const char* default_charset;
   const char* suggested_filename;
   const char* mime_type;
   const wchar_t* default_filename;
@@ -516,7 +516,7 @@
   std::string default_filename(WideToUTF8(test_case->default_filename));
   FilePath file_path = GenerateFileName(
       GURL(test_case->url), test_case->content_disp_header,
-      test_case->referrer_charset, test_case->suggested_filename,
+      test_case->default_charset, test_case->suggested_filename,
       test_case->mime_type, default_filename);
   EXPECT_EQ(test_case->expected_filename,
             file_util::FilePathAsWString(file_path))
@@ -2091,7 +2091,7 @@
 
   for (size_t i = 0; i < ARRAYSIZE_UNSAFE(generation_tests); ++i) {
     GenerateFilenameCase test_case = generation_tests[i];
-    test_case.referrer_charset = "GBK";
+    test_case.default_charset = "GBK";
     RunGenerateFileNameTestCase(&test_case, i, "generation (referrer=GBK)");
   }
 }