Make the default ctor of rtc::Thread, protected

webrtc/rtc_base/criticalsection_unittest.cc

Index: webrtc/rtc_base/criticalsection_unittest.cc
diff --git a/webrtc/rtc_base/criticalsection_unittest.cc b/webrtc/rtc_base/criticalsection_unittest.cc
index 672a6746a669924409beb839b17094c251fe11a0..2e136bf80418a3837837caf0bd9ee0fb6bd7eaf9 100644
--- a/webrtc/rtc_base/criticalsection_unittest.cc
+++ b/webrtc/rtc_base/criticalsection_unittest.cc
@@ -201,7 +201,7 @@ struct CompareAndSwapOp {
 void StartThreads(std::vector<std::unique_ptr<Thread>>* threads,
                   MessageHandler* handler) {
   for (int i = 0; i < kNumThreads; ++i) {
-    std::unique_ptr<Thread> thread(new Thread());
+    std::unique_ptr<Thread> thread(Thread::Create());
     thread->Start();
     thread->Post(RTC_FROM_HERE, handler);
     threads->push_back(std::move(thread));