Use typedef to shorten WorkerThreadableWebSocketChannel inner classes.

This CL doesn't change the behavior.

BUG=384238

Review URL: https://codereview.chromium.org/410393003

git-svn-id: svn://svn.chromium.org/blink/trunk@178813 bbb929c8-8fbe-4397-9dbb-9b2b20218538