Add a test for the empty ping case.

Verify that the WebSocket stack handles an empty ping correctly.

BUG=338290
TEST=run-webkits-tests.sh http/tests/websocket

Review URL: https://codereview.chromium.org/145693004

git-svn-id: svn://svn.chromium.org/blink/trunk@167009 bbb929c8-8fbe-4397-9dbb-9b2b20218538