Gardening: Mark http/tests/serviceworker/fetch.html as slow on Win7

NOTRY=true
TBR=arv@chromium.org
BUG=411779

Review URL: https://codereview.chromium.org/546633008

git-svn-id: svn://svn.chromium.org/blink/trunk@181540 bbb929c8-8fbe-4397-9dbb-9b2b20218538