Temporarily mark WebSocket throttling tests as flaky.

When Chromium CL https://codereview.chromium.org/240873003/ lands,
layout tests http/tests/websocket/multiple-connections.html and
http/tests/websocket/connection-throttling.html will start passing.

Temporarily mark them as flaky so they don't turn the tree red.

BUG=343107

Review URL: https://codereview.chromium.org/341983007

git-svn-id: svn://svn.chromium.org/blink/trunk@176825 bbb929c8-8fbe-4397-9dbb-9b2b20218538