Set |Channel::is_shutting_down_| during channel shutdown.

third_party/mojo/src/mojo/edk/system/channel.cc

Index: third_party/mojo/src/mojo/edk/system/channel.cc
diff --git a/third_party/mojo/src/mojo/edk/system/channel.cc b/third_party/mojo/src/mojo/edk/system/channel.cc
index 263f40b1473155c719fd773d689113d08ab023f7..1b46721ace2ce724b43e8c72f6798daf64b46cd8 100644
--- a/third_party/mojo/src/mojo/edk/system/channel.cc
+++ b/third_party/mojo/src/mojo/edk/system/channel.cc
@@ -69,6 +69,10 @@ void Channel::Shutdown() {
     // |OnReadMessage()| or |OnError()|.
     raw_channel_->Shutdown();
     is_running_ = false;
+    // |WillShutdownSoon()| may not have been called (i.e. on a channel error),
+    // and this flag is relied on to prevent a race with
+    // |AttachAndRunEndpoint()|.
+    is_shutting_down_ = true;
 
     // We need to deal with it outside the lock.
     std::swap(to_destroy, local_id_to_endpoint_map_);