Make the default ctor of rtc::Thread, protected

webrtc/rtc_base/thread.h

Index: webrtc/rtc_base/thread.h
diff --git a/webrtc/rtc_base/thread.h b/webrtc/rtc_base/thread.h
index 35b28eac5392593aadb799e62a8b803987fb4a87..72f15fde1cf8fbf3b8a3ab6531b58d4324f3c80b 100644
--- a/webrtc/rtc_base/thread.h
+++ b/webrtc/rtc_base/thread.h
@@ -101,9 +101,10 @@ class Runnable {
 // WARNING! SUBCLASSES MUST CALL Stop() IN THEIR DESTRUCTORS!  See ~Thread().
 
 class LOCKABLE Thread : public MessageQueue {
- public:
-  // Create a new Thread and optionally assign it to the passed SocketServer.
+ protected:
   Thread();
+
+ public:
   explicit Thread(SocketServer* ss);
   explicit Thread(std::unique_ptr<SocketServer> ss);