Gardening: Flaky timeout of http/tests/xmlhttprequest/origin-exact-matching.html

BUG=310882
TBR=antonm@chromium.org

Review URL: https://codereview.chromium.org/38923002

git-svn-id: svn://svn.chromium.org/blink/trunk@160398 bbb929c8-8fbe-4397-9dbb-9b2b20218538