Fix non-Mac 10.6 rebaseline of XHR test

BUG=
TBR=mkwst@chromium.org

Review URL: https://codereview.chromium.org/171533002

git-svn-id: svn://svn.chromium.org/blink/trunk@167372 bbb929c8-8fbe-4397-9dbb-9b2b20218538