Layout test for connection throttling.

Makes two WebSocket connections at the same time, verifies that the second
connection doesn't enter the handshake phase while the first connection is in
there. The handshake phase contains a delay of 0.05 seconds, which is hopefully
long enough to give the correct answer without flakiness.

BUG=343107

Review URL: https://codereview.chromium.org/201203003

git-svn-id: svn://svn.chromium.org/blink/trunk@174788 bbb929c8-8fbe-4397-9dbb-9b2b20218538