Timeout expectation for serviceworker/fetch-mixed-content.html

Timing out following chromium r300644, but this looks like a test-only
failure due to invalid cert in the localhost HTTPS server.

BUG=426047
TBR=aboxhall@chromium.org,horo@chromium.org

Review URL: https://codereview.chromium.org/668773003

git-svn-id: svn://svn.chromium.org/blink/trunk@184199 bbb929c8-8fbe-4397-9dbb-9b2b20218538