Gardening: http/tests/loading/script-priorities.html is flaky

BUG=413604
TBR= pmeenan@chromium.org, jianli@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/551543003

git-svn-id: svn://svn.chromium.org/blink/trunk@181892 bbb929c8-8fbe-4397-9dbb-9b2b20218538