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

net/http/http_auth_sspi_win.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.
 
 // This file contains common routines used by NTLM and Negotiate authentication
 // using the SSPI API on Windows.
 
 #ifndef NET_HTTP_HTTP_AUTH_SSPI_WIN_H_
 #define NET_HTTP_HTTP_AUTH_SSPI_WIN_H_
 #pragma once
@@ skipping 114 matching lines @@
 class NET_EXPORT_PRIVATE HttpAuthSSPI {
  public:
   HttpAuthSSPI(SSPILibrary* sspi_library,
                const std::string& scheme,
                SEC_WCHAR* security_package,
                ULONG max_token_length);
   ~HttpAuthSSPI();
 
   bool NeedsIdentity() const;
 
+  bool AllowsExplicitCredentials() const;
+
   HttpAuth::AuthorizationResult ParseChallenge(
       HttpAuth::ChallengeTokenizer* tok);
 
   // Generates an authentication token for the service specified by the
   // Service Principal Name |spn| and stores the value in |*auth_token|.
   // If the return value is not |OK|, then the value of |*auth_token| is
   // unspecified. ERR_IO_PENDING is not a valid return code.
   // If this is the first round of a multiple round scheme, credentials are
   // obtained using |*username| and |*password|. If |username| and |password|
   // are both NULL, the credentials for the currently logged in user are used
@@ skipping 54 matching lines @@
 // If the return value is ERR_UNEXPECTED, there was an unanticipated problem
 // in the underlying SSPI call. The details are logged, and |*max_token_length|
 // is not changed.
 NET_EXPORT_PRIVATE int DetermineMaxTokenLength(SSPILibrary* library,
                                                const std::wstring& package,
                                                ULONG* max_token_length);
 
 }  // namespace net
 
 #endif  // NET_HTTP_HTTP_AUTH_SSPI_WIN_H_