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

net/http/http_auth_sspi_win.cc

Index: net/http/http_auth_sspi_win.cc
diff --git a/net/http/http_auth_sspi_win.cc b/net/http/http_auth_sspi_win.cc
index 1a629f167ed1d43cf1e161f429f8325126b55af2..d07ce4ce19fb07386f51b455432e42ce6f2bf0af 100644
--- a/net/http/http_auth_sspi_win.cc
+++ b/net/http/http_auth_sspi_win.cc
@@ -1,4 +1,4 @@
-// Copyright (c) 2010 The Chromium Authors. All rights reserved.
+// 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.
 
@@ -210,6 +210,10 @@ bool HttpAuthSSPI::NeedsIdentity() const {
   return decoded_server_auth_token_.empty();
 }
 
+bool HttpAuthSSPI::AllowsExplicitCredentials() const {
+  return true;
+}
+
 void HttpAuthSSPI::Delegate() {
   can_delegate_ = true;
 }