Mark http/tests/media/gc-while-network-loading.html as slow

The test has slow runs of 3 seconds on Linux and Mac, and occasionally
times out at 6 seconds on Windows.

A more sophisticated server-side script could serve the first half of
the file very quickly and then trickle the rest.

BUG=400659
NOTRY=true
R=fs@opera.com

Review URL: https://codereview.chromium.org/591313003

git-svn-id: svn://svn.chromium.org/blink/trunk@182483 bbb929c8-8fbe-4397-9dbb-9b2b20218538