[mojo-edk] Fix bootstrap NodeChannel Start/ShutDown race

mojo/edk/system/node_channel.cc

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "mojo/edk/system/node_channel.h"
 
 #include <cstring>
 #include <limits>
 #include <sstream>
 
@@ skipping 154 matching lines @@
 }
 
 void NodeChannel::Start() {
 #if defined(OS_MACOSX) && !defined(OS_IOS)
   MachPortRelay* relay = delegate_->GetMachPortRelay();
   if (relay)
     relay->AddObserver(this);
 #endif
 
   base::AutoLock lock(channel_lock_);
-  DCHECK(channel_);
-  channel_->Start();
+  // ShutDown() may have already been called, in which case |channel_| is null.
+  if (channel_)
+    channel_->Start();
 }
 
 void NodeChannel::ShutDown() {
 #if defined(OS_MACOSX) && !defined(OS_IOS)
   MachPortRelay* relay = delegate_->GetMachPortRelay();
   if (relay)
     relay->RemoveObserver(this);
 #endif
 
   base::AutoLock lock(channel_lock_);
@@ skipping 557 matching lines @@
 
   base::AutoLock lock(channel_lock_);
   if (!channel_)
     DLOG(ERROR) << "Dropping message on closed channel.";
   else
     channel_->Write(std::move(message));
 }
 
 }  // namespace edk
 }  // namespace mojo