Mojo C++ bindings: handle double peer endpoint closed event for the same endpoint.

BUG=None

Review-Url: https://codereview.chromium.org/2037793003
Cr-Commit-Position: refs/heads/master@{#397811}

mojo/public/cpp/bindings/lib/multiplex_router.cc

@@ -526,11 +526,18 @@ bool MultiplexRouter::OnPeerAssociatedEndpointClosed(InterfaceId id) {
     return false;
 
   InterfaceEndpoint* endpoint = FindOrInsertEndpoint(id, nullptr);
-  DCHECK(!endpoint->peer_closed());
 
-  if (endpoint->client())
-    tasks_.push_back(Task::CreateNotifyErrorTask(endpoint));
-  UpdateEndpointStateMayRemove(endpoint, PEER_ENDPOINT_CLOSED);
+  // It is possible that this endpoint has been set as peer closed. That is
+  // because when the message pipe is closed, all the endpoints are updated with
+  // PEER_ENDPOINT_CLOSED. We continue to process remaining tasks in the queue,
+  // as long as there are refs keeping the router alive. If there is a
+  // PeerAssociatedEndpointClosedEvent control message in the queue, we will get
+  // here and see that the endpoint has been marked as peer closed.
+  if (!endpoint->peer_closed()) {
+    if (endpoint->client())
+      tasks_.push_back(Task::CreateNotifyErrorTask(endpoint));
+    UpdateEndpointStateMayRemove(endpoint, PEER_ENDPOINT_CLOSED);
+  }
 
   // No need to trigger a ProcessTasks() because it is already on the stack.