[XHR] Fix a XHR-stream test script.

LayoutTests/http/tests/xmlhttprequest/resources/slow-failure.cgi

 #!/usr/bin/perl
 
+# This script outputs a broken transfer-encoded response. In order to deliver
+# didReceiveResponse to the client (i.e. XHR), we send a small valid chunk
+# first, wait a while and then send an invalid chunk.
+
 use IO::Socket;
 
 $| = 1;
 
 autoflush STDOUT 1;
 
 print "Content-Type: text/html; charset=us-ascii\n";
+print "X-Content-Type-Options: nosniff\n";
 print "Transfer-encoding: chunked\n\n";
 
+print "5\nhello\n";
 sleep 2;
 
 print "hoge\nfuga\n";