Revert "Make freeNativePeerConnectionObserver generic."

This reverts commit 046f78cae64fec756391e81206c5aa007274b791.

Reason for revert: Breaks chromium.webrtc.fyi tree

Original change's description:
> Make freeNativePeerConnectionObserver generic.
> 
> Previously, it was only possible to free PeerConnectionObserverJni
> objects using this method. Now it is generic and can free any
> PeerConnectionObserver.
> 
> Bug: webrtc:8662
> Change-Id: I619ca5ed88a0c2553fa6d19ce41e510947d5bd44
> Reviewed-on: https://webrtc-review.googlesource.com/35222
> Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
> Reviewed-by: Magnus Jedvert <magjed@webrtc.org>
> Commit-Queue: Sami Kalliomäki <sakal@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#21411}

TBR=magjed@webrtc.org,sakal@webrtc.org,kwiberg@webrtc.org

Change-Id: I4490945ca3d9a25d5ed5795bc7954dc1044bdd22
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: webrtc:8662
Reviewed-on: https://webrtc-review.googlesource.com/35781
Reviewed-by: Sami Kalliomäki <sakal@webrtc.org>
Commit-Queue: Sami Kalliomäki <sakal@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21413}

api/peerconnectioninterface.h

@@ -973,8 +973,6 @@ class PeerConnectionObserver {
     kIceState,
   };
 
-  virtual ~PeerConnectionObserver() = default;
-
   // Triggered when the SignalingState changed.
   virtual void OnSignalingChange(
       PeerConnectionInterface::SignalingState new_state) = 0;
@@ -1047,6 +1045,10 @@ class PeerConnectionObserver {
   // TODO(hbos,deadbeef): Make pure virtual when all subclasses implement it.
   virtual void OnRemoveTrack(
       rtc::scoped_refptr<RtpReceiverInterface> receiver) {}
+
+ protected:
+  // Dtor protected as objects shouldn't be deleted via this interface.
+  ~PeerConnectionObserver() {}
 };
 
 // PeerConnectionFactoryInterface is the factory interface used for creating

sdk/android/src/jni/pc/peerconnection.cc

@@ -329,7 +329,8 @@ static void JNI_PeerConnection_FreePeerConnectionObserver(
     JNIEnv*,
     const JavaParamRef<jclass>&,
     jlong j_p) {
-  PeerConnectionObserver* p = reinterpret_cast<PeerConnectionObserver*>(j_p);
+  PeerConnectionObserverJni* p =
+      reinterpret_cast<PeerConnectionObserverJni*>(j_p);
   delete p;
 }