Fix failing DCHECKs in JingleSession added in r294653

r294653 added two DCHECKs in JingleSession in CancelChannelCreation()
and destructor. These checks were failing in the client sometimes. This
CL fixes the code that was causing them to fail:
 1. The ChannelMultiplexer needs to be destroyed explicitly in the
    destructor.
 2. ChannelDispatcherBase was calling CancelChannelCreation() even
    after the channel is created.

Review URL: https://codereview.chromium.org/571273003

Cr-Commit-Position: refs/heads/master@{#295138}

remoting/protocol/channel_dispatcher_base.cc

@@ -55,6 +55,7 @@ void ChannelDispatcherBase::OnChannelReady(
     return;
   }
 
+  channel_factory_ = NULL;
   channel_ = socket.Pass();
 
   OnInitialized();

remoting/protocol/jingle_session.cc

@@ -84,7 +84,10 @@ JingleSession::~JingleSession() {
                              pending_requests_.end());
   STLDeleteContainerPointers(transport_info_requests_.begin(),
                              transport_info_requests_.end());
+
+  channel_multiplexer_.reset();
   DCHECK(channels_.empty());
+
   session_manager_->SessionDestroyed(this);
 }