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

net/http/http_auth_gssapi_posix.cc

Index: net/http/http_auth_gssapi_posix.cc
diff --git a/net/http/http_auth_gssapi_posix.cc b/net/http/http_auth_gssapi_posix.cc
index cc27e70b18af1375bdb7fcd28d83fb3339a122cf..1703b2d2b1d25eb3ea0869960b7305701ea3633d 100644
--- a/net/http/http_auth_gssapi_posix.cc
+++ b/net/http/http_auth_gssapi_posix.cc
@@ -676,6 +676,10 @@ bool HttpAuthGSSAPI::NeedsIdentity() const {
   return decoded_server_auth_token_.empty();
 }
 
+bool HttpAuthGSSAPI::AllowsExplicitCredentials() const {
+  return false;
+}
+
 void HttpAuthGSSAPI::Delegate() {
   can_delegate_ = true;
 }