Propogate network-worker thread split to api

BUG=webrtc:5645

Review-Url: https://codereview.webrtc.org/1968393002
Cr-Commit-Position: refs/heads/master@{#12767}
diff --git a/webrtc/api/peerconnectioninterface_unittest.cc b/webrtc/api/peerconnectioninterface_unittest.cc
index d33a402..445f025 100644
--- a/webrtc/api/peerconnectioninterface_unittest.cc
+++ b/webrtc/api/peerconnectioninterface_unittest.cc
@@ -545,9 +545,9 @@
 
   virtual void SetUp() {
     pc_factory_ = webrtc::CreatePeerConnectionFactory(
-        rtc::Thread::Current(), rtc::Thread::Current(), NULL, NULL,
-        NULL);
-    ASSERT_TRUE(pc_factory_.get() != NULL);
+        rtc::Thread::Current(), rtc::Thread::Current(), rtc::Thread::Current(),
+        nullptr, nullptr, nullptr);
+    ASSERT_TRUE(pc_factory_);
   }
 
   void CreatePeerConnection() {