Reorder setting passthrough flag to true to happen only after draining the pending messages for sendingMessage.

Author: kannanjgithub

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

@@ -1080,11 +1080,11 @@ private void handleImmediateResponse(ImmediateResponse immediate)
 
     private void drainPendingDrainingMessages() {
       synchronized (rawCallLock) {
-        passThroughMode.set(true);
         InputStream msg;
         while ((msg = pendingDrainingMessages.poll()) != null) {
           super.sendMessage(msg);
         }
+        passThroughMode.set(true);
       }
     }