[LayoutTest] Fix xmlhttprequest-responseXML-nopartial flakiness.

xmlhttprequest-responseXML-nopartial used resources/small-chunks.cgi that takes
5 seconds to complete and it led to flaky timeout. Let's use a usual response.

BUG=432784
R=kouhei

Review URL: https://codereview.chromium.org/731443002

git-svn-id: svn://svn.chromium.org/blink/trunk@185364 bbb929c8-8fbe-4397-9dbb-9b2b20218538