Fix issue with deprecated CongestionController interface not working.

BUG=b/33446014

Review-Url: https://codereview.webrtc.org/2565503002
Cr-Original-Commit-Position: refs/heads/master@{#15491}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 7aa9b910ff0bc97fd9f2f1bf296ed8e4554862a2
diff --git a/modules/congestion_controller/include/congestion_controller.h b/modules/congestion_controller/include/congestion_controller.h
index e1151c1..004a847 100644
--- a/modules/congestion_controller/include/congestion_controller.h
+++ b/modules/congestion_controller/include/congestion_controller.h
@@ -47,17 +47,10 @@
   // packet headers and is measured in bits per second.
   class Observer {
    public:
-    // This is to be deprecated. Please use the one below instead.
-    RTC_DEPRECATED virtual void OnNetworkChanged(uint32_t bitrate_bps,
-                                                 uint8_t fraction_loss,
-                                                 int64_t rtt_ms) {}
-
-    // TODO(minyue): make this method pure virtual when all uses of the old API
-    // have been updated.
     virtual void OnNetworkChanged(uint32_t bitrate_bps,
                                   uint8_t fraction_loss,  // 0 - 255.
                                   int64_t rtt_ms,
-                                  int64_t probing_interval_ms) {}
+                                  int64_t probing_interval_ms) = 0;
 
    protected:
     virtual ~Observer() {}