Avoid data race in PeerConnectionDependencyFactory.

InitializeSignalingThread reads from `worker_thread_`, without
synchronizing with InitializeWorkerThread which writes it.

This CL moves the wait for IWT to before IST can run.

Bug: 1160469
Change-Id: I9fd5f6341f900c1b9d43386c427b4086333e6f1e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2601044
Auto-Submit: Frank Liberato <liberato@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Kinuko Yasuda <kinuko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#839028}

third_party/blink/renderer/modules/peerconnection/peer_connection_dependency_factory.cc

@@ -242,6 +242,13 @@ void PeerConnectionDependencyFactory::CreatePeerConnectionFactory() {
   create_network_manager_event.Wait();
   CHECK(network_thread_);
 
+  // Wait for the worker thread, since `InitializeSignalingThread` needs to
+  // refer to `worker_thread_`.
+  if (chrome_worker_thread_) {
+    start_worker_event.Wait();
+    CHECK(worker_thread_);
+  }
+
   base::WaitableEvent start_signaling_event(
       base::WaitableEvent::ResetPolicy::MANUAL,
       base::WaitableEvent::InitialState::NOT_SIGNALED);
@@ -254,10 +261,6 @@ void PeerConnectionDependencyFactory::CreatePeerConnectionFactory() {
           CrossThreadUnretained(Platform::Current()->GetMediaDecoderFactory()),
           CrossThreadUnretained(&start_signaling_event)));
 
-  if (chrome_worker_thread_) {
-    start_worker_event.Wait();
-    CHECK(worker_thread_);
-  }
   start_signaling_event.Wait();
   CHECK(signaling_thread_);
 }