Use WeakPtr in WorkerThreadableWebSocketChannel

As a preparation for getting rid of runInMode (and for cleanup)

BUG=340249
TEST=http/tests/websocket/workers/*

Review URL: https://codereview.chromium.org/151293004

git-svn-id: svn://svn.chromium.org/blink/trunk@166377 bbb929c8-8fbe-4397-9dbb-9b2b20218538