Cleanup return values in http_auth

net/http/http_auth_handler.h

Index: net/http/http_auth_handler.h
diff --git a/net/http/http_auth_handler.h b/net/http/http_auth_handler.h
index 439fe076b085dc7deafee6ada0fec467be6f8654..1aee79556b0502e86929e625259932c379573ace 100644
--- a/net/http/http_auth_handler.h
+++ b/net/http/http_auth_handler.h
@@ -87,9 +87,7 @@ class NET_EXPORT_PRIVATE HttpAuthHandler {
   }
 
   // The challenge which was issued when creating the handler.
-  const std::string challenge() const {
-    return auth_challenge_;
-  }
+  const std::string& challenge() const { return auth_challenge_; }
 
   // Numeric rank based on the challenge's security level. Higher
   // numbers are better. Used by HttpAuth::ChooseBestChallenge().