Add a method for getting the native factory from PeerConnectionFactory.

Bug: webrtc:8662
Change-Id: Icfe09e82737e1b700e469afde00c4f0923bbeb42
Reviewed-on: https://webrtc-review.googlesource.com/39516
Reviewed-by: Anders Carlsson <andersc@webrtc.org>
Commit-Queue: Sami Kalliomäki <sakal@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21628}

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

@@ -490,5 +490,12 @@ static void JNI_PeerConnectionFactory_SetVideoHwAccelerationOptions(
                 remote_egl_context);
 }
 
+static jlong JNI_PeerConnectionFactory_GetNativePeerConnectionFactory(
+    JNIEnv* jni,
+    const JavaParamRef<jclass>&,
+    jlong native_factory) {
+  return jlongFromPointer(factoryFromJava(native_factory));
+}
+
 }  // namespace jni
 }  // namespace webrtc