Fixing memory leak of generated session descriptions on Android.

SessionDescriptionObserver::OnSuccess takes ownership of the
description, but it doesn't use a unique_ptr, so the JNI code was
neglecting to delete it.

BUG=webrtc:7680

Review-Url: https://codereview.webrtc.org/2886263003
Cr-Commit-Position: refs/heads/master@{#18200}

webrtc/sdk/android/src/jni/peerconnection_jni.cc

@@ -658,6 +658,9 @@ class SdpObserverWrapper : public T {
     jobject j_sdp = JavaSdpFromNativeSdp(jni(), desc);
     jni()->CallVoidMethod(*j_observer_global_, m, j_sdp);
     CHECK_EXCEPTION(jni()) << "error during CallVoidMethod";
+    // OnSuccess transfers ownership of the description (there's a TODO to make
+    // it use unique_ptr...).
+    delete desc;
   }
 
  protected: