Check given ThreadableWebSocketChannelClientWrapper is not null only in the constructors

ThreadableWebSocketChannelClientWrapper is never cleared in the
WorkerThreadableWebSocketChannel or any of its subclasses. Checking
m_workerClientWrapper in every method is just confusing. Remove them
and make the member variables const RefPtr.

BUG=none

Review URL: https://codereview.chromium.org/257353002

git-svn-id: svn://svn.chromium.org/blink/trunk@173061 bbb929c8-8fbe-4397-9dbb-9b2b20218538