Add http/tests/inspector/network/long-script-content.html to SlowTests

TBR=mkwst@chromium.org
BUG=327078

Review URL: https://codereview.chromium.org/93793006

git-svn-id: svn://svn.chromium.org/blink/trunk@163453 bbb929c8-8fbe-4397-9dbb-9b2b20218538