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

net/http/http_auth_handler_negotiate.cc

Index: net/http/http_auth_handler_negotiate.cc
diff --git a/net/http/http_auth_handler_negotiate.cc b/net/http/http_auth_handler_negotiate.cc
index 52a2b1e60db323433dccd0315b49a8935fec07b7..d5d0f806d341fc81da32a64310ec203a834836b2 100644
--- a/net/http/http_auth_handler_negotiate.cc
+++ b/net/http/http_auth_handler_negotiate.cc
@@ -182,6 +182,10 @@ bool HttpAuthHandlerNegotiate::AllowsDefaultCredentials() {
   return url_security_manager_->CanUseDefaultCredentials(origin_);
 }
 
+bool HttpAuthHandlerNegotiate::AllowsExplicitCredentials() {
+  return auth_system_.AllowsExplicitCredentials();
+}
+
 // The Negotiate challenge header looks like:
 //   WWW-Authenticate: NEGOTIATE auth-data
 bool HttpAuthHandlerNegotiate::Init(HttpAuth::ChallengeTokenizer* challenge) {