Fix potential teardown race conditions with TCPEngineTransport's PostTasks.

blimp/net/tcp_engine_transport.h

Index: blimp/net/tcp_engine_transport.h
diff --git a/blimp/net/tcp_engine_transport.h b/blimp/net/tcp_engine_transport.h
index b7e85e06b83ec98e60d4439e80dd710370d0c846..4ab48304538e27010067e415df3bab25def29ef0 100644
--- a/blimp/net/tcp_engine_transport.h
+++ b/blimp/net/tcp_engine_transport.h
@@ -49,6 +49,7 @@ class BLIMP_NET_EXPORT TCPEngineTransport : public BlimpEngineTransport {
   std::unique_ptr<net::ServerSocket> server_socket_;
   std::unique_ptr<net::StreamSocket> accepted_socket_;
   net::CompletionCallback connect_callback_;
+  base::WeakPtrFactory<TCPEngineTransport> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(TCPEngineTransport);
 };