[WebSocket] Skip a flaky layout test.

The implementation will be fixed later.
See http://crbug.com/339373 .

BUG=340445
R=tyoshino

Review URL: https://codereview.chromium.org/144663004

git-svn-id: svn://svn.chromium.org/blink/trunk@166362 bbb929c8-8fbe-4397-9dbb-9b2b20218538