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

net/http/http_auth_handler.cc

Index: net/http/http_auth_handler.cc
diff --git a/net/http/http_auth_handler.cc b/net/http/http_auth_handler.cc
index 51bd6aabf18a42b58bd67a4089565c513d531186..d123c21aa601ae3266dd84f9c247512781b4d3a6 100644
--- a/net/http/http_auth_handler.cc
+++ b/net/http/http_auth_handler.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.
 
@@ -90,6 +90,10 @@ bool HttpAuthHandler::AllowsDefaultCredentials() {
   return false;
 }
 
+bool HttpAuthHandler::AllowsExplicitCredentials() {
+  return true;
+}
+
 void HttpAuthHandler::OnGenerateAuthTokenComplete(int rv) {
   CompletionCallback* callback = original_callback_;
   FinishGenerateAuthToken();