Fix WebSocket crash bug.

      If SocketStream closes while waiting ResolveProxy, it badly
calls DoResolveProxyComplete from DoLoop invoked by SocketStream::Close,
and real callback of ResolveProxy failed to call SocketStream::OnIOCompleted.

So, don't run DoLoop when closed, if SocketStream is calling APIs of
proxy_service or resolver.  Check closing_ after the API calls back
and closes the SocketStream.

BUG=50394,46750
TEST=websocket/tests doesn't crash

Review URL: http://codereview.chromium.org/3054039

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@54707 0039d316-1c4b-4281-b951-d872f2087c98