Rename NewWebSocketChannelImpl to DocumentWebSocketChannel.

As the new WebSocket implementation is used as default and the old one was
deleted, NewWebSocketChannelImpl is not an appropriate name.

BUG=423201
R=ricea@chromium.org

Review URL: https://codereview.chromium.org/690883002

git-svn-id: svn://svn.chromium.org/blink/trunk@184817 bbb929c8-8fbe-4397-9dbb-9b2b20218538