Partial Reland of Make the default ctor of rtc::Thread, protected

webrtc/rtc_base/socket_unittest.cc

Index: webrtc/rtc_base/socket_unittest.cc
diff --git a/webrtc/rtc_base/socket_unittest.cc b/webrtc/rtc_base/socket_unittest.cc
index 35bf2d1c4dc9a6151341c34fe2f04b8f19e62adf..5ef01679ffbde42de60dcb8b6a82128c52565392 100644
--- a/webrtc/rtc_base/socket_unittest.cc
+++ b/webrtc/rtc_base/socket_unittest.cc
@@ -681,7 +681,7 @@ void SocketTest::SocketServerWaitInternal(const IPAddress& loopback) {
   EXPECT_FALSE(sink.Check(accepted.get(), SSE_READ));
 
   // Shouldn't signal when blocked in a thread Send, where process_io is false.
-  std::unique_ptr<Thread> thread(new Thread());
+  std::unique_ptr<Thread> thread(Thread::CreateWithSocketServer());
   thread->Start();
   Sleeper sleeper;
   TypedMessageData<AsyncSocket*> data(client.get());