Removed observer from probe controller.

Replacing observer interface with polling for pending probe clusters.
The purpose is to make it easier to reason about and control side
effects and to prepare for a similar change in the network controller
interface.

Bug: webrtc:8415
Change-Id: I8101cfda22e640a8e0fa75f3f6e63876db826a89
Reviewed-on: https://webrtc-review.googlesource.com/66881
Commit-Queue: Sebastian Jansson <srte@webrtc.org>
Reviewed-by: Philip Eliasson <philipel@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22775}

modules/congestion_controller/rtp/send_side_congestion_controller.cc

@@ -348,6 +348,7 @@ void SendSideCongestionController::MaybeCreateControllers() {
   controller_ =
       controller_factory_->Create(control_handler_.get(), initial_config_);
   UpdateStreamsConfig();
+  UpdateControllerWithTimeInterval();
   StartProcessPeriodicTasks();
 }