Fix incorrect behavior when handling draining of ext proc request messages queue. Passthough flag should only be set to true afetr this draining is complete, as otherwise application.sendMessage will deliver the message upstream out of order.

Author: kannanjgithub

xds/src/main/java/io/grpc/xds/ExternalProcessorFilter.java

@@ -1512,17 +1512,16 @@ private void handleImmediateResponse(ImmediateResponse immediate, DataPlaneListe
 
       private void drainPendingDrainingMessages() {
         synchronized (streamLock) {
-          passThroughMode.set(true);
           InputStream msg;
           while ((msg = pendingDrainingMessages.poll()) != null) {
             super.sendMessage(msg);
           }
+          passThroughMode.set(true);
         }
       }
 
       private void handleFailOpen(DataPlaneListener listener) {
         activateCall();
-        drainPendingDrainingMessages();
         listener.unblockAfterStreamComplete();
         closeExtProcStream();
       }