Make the default ctor of rtc::Thread, protected.

The goal is to force use of Thread::Create or Thread::CreateWithSocketServer.

The default constructor constructs a 'default' socket server, which is usually a 'physical' socket server, but not always. Not every instance of Thread actually needs to have network support, so it's better to have this be explicit instead of unknowingly instantiate one.

BUG=none

Review-Url: https://codereview.webrtc.org/2981623002
Cr-Commit-Position: refs/heads/master@{#19001}

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());