Make TaskQueueFactory required construction parameter for Call

Bug: webrtc:10284
Change-Id: I573ee0087c035e26918260c21b8b0213ddfe7ebc
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/143791
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Reviewed-by: Niels Moller <nisse@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28467}
diff --git a/call/call_config.h b/call/call_config.h
index d111b53..a6f7a96 100644
--- a/call/call_config.h
+++ b/call/call_config.h
@@ -47,7 +47,7 @@
   // FecController to use for this call.
   FecControllerFactoryInterface* fec_controller_factory = nullptr;
 
-  // Task Queue Factory to be used in this call.
+  // Task Queue Factory to be used in this call. Required.
   TaskQueueFactory* task_queue_factory = nullptr;
 
   // NetworkStatePredictor to use for this call.