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

webrtc/rtc_base/thread.h

 /*
  *  Copyright 2004 The WebRTC Project Authors. All rights reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 84 matching lines @@
   Runnable() {}
 
  private:
   RTC_DISALLOW_COPY_AND_ASSIGN(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.
+  // DEPRECATED.
+  // The default constructor should not be used because it hides whether or
+  // not a socket server will be associated with the thread. Most instances
+  // of Thread do actually not need one, so please use either of the Create*
+  // methods to construct an instance of Thread.
   Thread();
+
   explicit Thread(SocketServer* ss);
   explicit Thread(std::unique_ptr<SocketServer> ss);
 
   // NOTE: ALL SUBCLASSES OF Thread MUST CALL Stop() IN THEIR DESTRUCTORS (or
   // guarantee Stop() is explicitly called before the subclass is destroyed).
   // This is required to avoid a data race between the destructor modifying the
   // vtable, and the Thread::PreRun calling the virtual method Run().
   ~Thread() override;
 
   static std::unique_ptr<Thread> CreateWithSocketServer();
@@ skipping 206 matching lines @@
 
  private:
   rtc::Thread* old_thread_;
 
   RTC_DISALLOW_COPY_AND_ASSIGN(AutoSocketServerThread);
 };
 
 }  // namespace rtc
 
 #endif  // WEBRTC_RTC_BASE_THREAD_H_