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

net/http/http_auth_gssapi_posix.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_GSSAPI_POSIX_H_
 #define NET_HTTP_HTTP_AUTH_GSSAPI_POSIX_H_
 #pragma once
 
 #include <gssapi.h>
 
@@ skipping 216 matching lines @@
  public:
   HttpAuthGSSAPI(GSSAPILibrary* library,
                  const std::string& scheme,
                  const gss_OID gss_oid);
   ~HttpAuthGSSAPI();
 
   bool Init();
 
   bool NeedsIdentity() const;
 
+  bool AllowsExplicitCredentials() const;
+
   HttpAuth::AuthorizationResult ParseChallenge(
       HttpAuth::ChallengeTokenizer* tok);
 
   // Generates an authentication token.
   // The return value is an error code. If it's not |OK|, the value of
   // |*auth_token| is unspecified.
   // |spn| is the Service Principal Name of the server that the token is
   // being generated for.
   // If this is the first round of a multiple round scheme, credentials are
   // obtained using |*username| and |*password|. If |username| and |password|
@@ skipping 17 matching lines @@
   gss_OID gss_oid_;
   GSSAPILibrary* library_;
   std::string decoded_server_auth_token_;
   ScopedSecurityContext scoped_sec_context_;
   bool can_delegate_;
 };
 
 }  // namespace net
 
 #endif  // NET_HTTP_HTTP_AUTH_GSSAPI_POSIX_H_