Reduce scope (and running time) of sync-repeated-and-caching test.

Synchronous XMLHttpRequest fetches are slow end-to-end when running
LayoutTests, particularly on Windows. The range of requests issued by
xmlhttprequest/sync-repeated-and-caching really doesn't have to be
that extensive to catch regressions. Hence, reduce scope somewhat.

R=tyoshino@chromium.org
BUG=327877

Review URL: https://codereview.chromium.org/150103005

git-svn-id: svn://svn.chromium.org/blink/trunk@166615 bbb929c8-8fbe-4397-9dbb-9b2b20218538