Don't try to use explicit credentials with schemes that don't support it.

net/http/http_auth_handler_negotiate.h

 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef NET_HTTP_HTTP_AUTH_HANDLER_NEGOTIATE_H_
 #define NET_HTTP_HTTP_AUTH_HANDLER_NEGOTIATE_H_
 #pragma once
 
 #include <string>
 
@@ skipping 90 matching lines @@
                            bool use_port);
 
   virtual ~HttpAuthHandlerNegotiate();
 
   // These are public for unit tests
   std::wstring CreateSPN(const AddressList& address_list, const GURL& orign);
   const std::wstring& spn() const { return spn_; }
 
   // HttpAuthHandler:
   virtual HttpAuth::AuthorizationResult HandleAnotherChallenge(
-      HttpAuth::ChallengeTokenizer* challenge);
-  virtual bool NeedsIdentity();
-  virtual bool AllowsDefaultCredentials();
+      HttpAuth::ChallengeTokenizer* challenge) OVERRIDE;
+  virtual bool NeedsIdentity() OVERRIDE;
+  virtual bool AllowsDefaultCredentials() OVERRIDE;
+  virtual bool AllowsExplicitCredentials() OVERRIDE;
 
  protected:
   virtual bool Init(HttpAuth::ChallengeTokenizer* challenge);
 
   virtual int GenerateAuthTokenImpl(const string16* username,
                                     const string16* password,
                                     const HttpRequestInfo* request,
                                     CompletionCallback* callback,
                                     std::string* auth_token);
 
@@ skipping 38 matching lines @@
   std::string* auth_token_;
 
   State next_state_;
 
   const URLSecurityManager* url_security_manager_;
 };
 
 }  // namespace net
 
 #endif  // NET_HTTP_HTTP_AUTH_HANDLER_NEGOTIATE_H_