Add ability to set max probing bitrate in SendSideCongestionController.

While this class is deprecated, it's needed as a stop-gap solution.
Other methods to configure the max probe rate all effect the current
estimate and/or trigger new probes to be sent, and we need a way to
configure the max without affecting other behavior.

Bug: webrtc:10070
Change-Id: I2b0ba2fef42d0bab6e5ea7f7c921681557802b4b
Reviewed-on: https://webrtc-review.googlesource.com/c/114880
Reviewed-by: Sebastian Jansson <srte@webrtc.org>
Commit-Queue: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26162}

modules/congestion_controller/goog_cc/probe_controller.h

@@ -63,6 +63,9 @@ class ProbeController {
   RTC_WARN_UNUSED_RESULT std::vector<ProbeClusterConfig>
   InitiateCapacityProbing(int64_t bitrate_bps, int64_t at_time_ms);
 
+  // Sets a new maximum probing bitrate, without generating a new probe cluster.
+  void SetMaxBitrate(int64_t max_bitrate_bps);
+
   // Resets the ProbeController to a state equivalent to as if it was just
   // created EXCEPT for |enable_periodic_alr_probing_|.
   void Reset(int64_t at_time_ms);