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

net/url_request/url_request_context.h

Index: net/url_request/url_request_context.h
===================================================================
--- net/url_request/url_request_context.h	(revision 119865)
+++ net/url_request/url_request_context.h	(working copy)
@@ -177,11 +177,9 @@
   // method to provide a UA string.
   virtual const std::string& GetUserAgent(const GURL& url) const;
 
-  // In general, referrer_charset is not known when URLRequestContext is
-  // constructed. So, we need a setter.
-  const std::string& referrer_charset() const { return referrer_charset_; }
-  void set_referrer_charset(const std::string& charset) {
-    referrer_charset_ = charset;
+  const std::string& default_charset() const { return default_charset_; }
+  void set_default_charset(const std::string& charset) {
+    default_charset_ = charset;
   }
 
   const URLRequestJobFactory* job_factory() const { return job_factory_; }
@@ -219,10 +217,9 @@
   scoped_ptr<FtpAuthCache> ftp_auth_cache_;
   std::string accept_language_;
   std::string accept_charset_;
-  // The charset of the referrer where this request comes from. It's not
-  // used in communication with a server but is used to construct a suggested
-  // filename for file download.
-  std::string referrer_charset_;
+  // The user's default charset. It's not used in communication with a server
+  // but is used to construct a suggested filename for file download.
+  std::string default_charset_;
   HttpTransactionFactory* http_transaction_factory_;
   FtpTransactionFactory* ftp_transaction_factory_;
   const URLRequestJobFactory* job_factory_;