Makes protected destructor public in TargetTransferRateObserver.

Bug: webrtc:9155
Change-Id: Id459b18d0933e5b5be89df5f7547f080539d4508
Reviewed-on: https://webrtc-review.googlesource.com/73379
Commit-Queue: Sebastian Jansson <srte@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23097}

modules/congestion_controller/network_control/include/network_control.h

@@ -19,12 +19,10 @@ namespace webrtc {
 
 class TargetTransferRateObserver {
  public:
+  virtual ~TargetTransferRateObserver() = default;
   // Called to indicate target transfer rate as well as giving information about
   // the current estimate of network parameters.
   virtual void OnTargetTransferRate(TargetTransferRate) = 0;
-
- protected:
-  virtual ~TargetTransferRateObserver() = default;
 };
 
 // Configuration sent to factory create function. The parameters here are