Suppress events when stop() has been called.

Some methods called in WebSocket::stop() may lead to calling dispatchEvent()
and the event handlers set to handle the events may modify the set of
ActiveDOMObject instances. This crashes the renderer. If the reference count
of the WebSocket instance goes down to 0 by calling unsetPendingActivity(),
this also causes modification of the set of ActiveDOMObject instances

To prevent them, suppress any dispatchEvent() call using a flag.

BUG=298060

Review URL: https://codereview.chromium.org/25441002

git-svn-id: svn://svn.chromium.org/blink/trunk@158692 bbb929c8-8fbe-4397-9dbb-9b2b20218538