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

net/http/http_auth_handler.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_H_
 #define NET_HTTP_HTTP_AUTH_HANDLER_H_
 #pragma once
 
 #include <string>
 
@@ skipping 117 matching lines @@
   // sequence used by a connection-based authentication scheme.
   virtual bool NeedsIdentity();
 
   // Returns whether the default credentials may be used for the |origin| passed
   // into |InitFromChallenge|. If true, the user does not need to be prompted
   // for username and password to establish credentials.
   // NOTE: SSO is a potential security risk.
   // TODO(cbentzel): Add a pointer to Firefox documentation about risk.
   virtual bool AllowsDefaultCredentials();
 
+  // Returns whether explicit credentials can be used with this handler.  If
+  // true the user may be prompted for credentials if an implicit identity
+  // cannot be determined.
+  virtual bool AllowsExplicitCredentials();
+
  protected:
   enum Property {
     ENCRYPTS_IDENTITY = 1 << 0,
     IS_CONNECTION_BASED = 1 << 1,
   };
 
   // Initializes the handler using a challenge issued by a server.
   // |challenge| must be non-NULL and have already tokenized the
   // authentication scheme, but none of the tokens occuring after the
   // authentication scheme.
@@ skipping 39 matching lines @@
   void OnGenerateAuthTokenComplete(int rv);
   void FinishGenerateAuthToken();
 
   CompletionCallback* original_callback_;
   CompletionCallbackImpl<HttpAuthHandler> wrapper_callback_;
 };
 
 }  // namespace net
 
 #endif  // NET_HTTP_HTTP_AUTH_HANDLER_H_