Remove ThreadableWebSocketChannelClientWrapper.

This is a refactoring CL and doesn't change the behavior.

BUG=384238

Review URL: https://codereview.chromium.org/467383006

git-svn-id: svn://svn.chromium.org/blink/trunk@180510 bbb929c8-8fbe-4397-9dbb-9b2b20218538