[XHR] Fix a flaky LayoutTest.

inspector/network/network-xhr-cache-control.html is flaky because the result
depends on timing. This CL fixes the test.

BUG=333804
R=tyoshino

Review URL: https://codereview.chromium.org/142413007

git-svn-id: svn://svn.chromium.org/blink/trunk@166016 bbb929c8-8fbe-4397-9dbb-9b2b20218538