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

mojo/edk/system/node_channel.cc

Index: mojo/edk/system/node_channel.cc
diff --git a/mojo/edk/system/node_channel.cc b/mojo/edk/system/node_channel.cc
index e6c79c11ed8cd200d828bbff0cb82bdc67adab24..5cb1b881624a362cbaf910e7c28c020504dbf491 100644
--- a/mojo/edk/system/node_channel.cc
+++ b/mojo/edk/system/node_channel.cc
@@ -172,8 +172,9 @@ void NodeChannel::Start() {
 #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() {