Revert of Make the default ctor of rtc::Thread, protected (patchset #3 id:40001 of https://codereview.webrtc.org/2981623002/ )

Reason for revert:
Break projects.

Original issue's description:
> 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}
> Committed: https://chromium.googlesource.com/external/webrtc/+/a8a351599788f346a2265cfcd3da9a06f577de7f

TBR=kthelgason@webrtc.org,tommi@webrtc.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=none

Review-Url: https://codereview.webrtc.org/2979963002
Cr-Commit-Position: refs/heads/master@{#19003}
diff --git a/webrtc/rtc_base/messagequeue_unittest.cc b/webrtc/rtc_base/messagequeue_unittest.cc
index 4003e2d..e31adf9 100644
--- a/webrtc/rtc_base/messagequeue_unittest.cc
+++ b/webrtc/rtc_base/messagequeue_unittest.cc
@@ -38,9 +38,9 @@
   bool IsLocked() {
     // We have to do this on a worker thread, or else the TryEnter will
     // succeed, since our critical sections are reentrant.
-    std::unique_ptr<Thread> worker(Thread::CreateWithSocketServer());
-    worker->Start();
-    return worker->Invoke<bool>(
+    Thread worker;
+    worker.Start();
+    return worker.Invoke<bool>(
         RTC_FROM_HERE, rtc::Bind(&MessageQueueTest::IsLocked_Worker, this));
   }
 };
@@ -152,10 +152,10 @@
 // all registered message queues.
 TEST(MessageQueueManager, ProcessAllMessageQueues) {
   Event entered_process_all_message_queues(true, false);
-  auto a = Thread::CreateWithSocketServer();
-  auto b = Thread::CreateWithSocketServer();
-  a->Start();
-  b->Start();
+  Thread a;
+  Thread b;
+  a.Start();
+  b.Start();
 
   volatile int messages_processed = 0;
   FunctorMessageHandler<void, std::function<void()>> incrementer(
@@ -173,10 +173,10 @@
       });
 
   // Post messages (both delayed and non delayed) to both threads.
-  a->Post(RTC_FROM_HERE, &incrementer);
-  b->Post(RTC_FROM_HERE, &incrementer);
-  a->PostDelayed(RTC_FROM_HERE, 0, &incrementer);
-  b->PostDelayed(RTC_FROM_HERE, 0, &incrementer);
+  a.Post(RTC_FROM_HERE, &incrementer);
+  b.Post(RTC_FROM_HERE, &incrementer);
+  a.PostDelayed(RTC_FROM_HERE, 0, &incrementer);
+  b.PostDelayed(RTC_FROM_HERE, 0, &incrementer);
   rtc::Thread::Current()->Post(RTC_FROM_HERE, &event_signaler);
 
   MessageQueueManager::ProcessAllMessageQueues();
@@ -185,9 +185,9 @@
 
 // Test that ProcessAllMessageQueues doesn't hang if a thread is quitting.
 TEST(MessageQueueManager, ProcessAllMessageQueuesWithQuittingThread) {
-  auto t = Thread::CreateWithSocketServer();
-  t->Start();
-  t->Quit();
+  Thread t;
+  t.Start();
+  t.Quit();
   MessageQueueManager::ProcessAllMessageQueues();
 }
 
@@ -195,8 +195,8 @@
 // messages.
 TEST(MessageQueueManager, ProcessAllMessageQueuesWithClearedQueue) {
   Event entered_process_all_message_queues(true, false);
-  auto t = Thread::CreateWithSocketServer();
-  t->Start();
+  Thread t;
+  t.Start();
 
   FunctorMessageHandler<void, std::function<void()>> clearer(
       [&entered_process_all_message_queues] {
@@ -213,7 +213,7 @@
       });
 
   // Post messages (both delayed and non delayed) to both threads.
-  t->Post(RTC_FROM_HERE, &clearer);
+  t.Post(RTC_FROM_HERE, &clearer);
   rtc::Thread::Current()->Post(RTC_FROM_HERE, &event_signaler);
   MessageQueueManager::ProcessAllMessageQueues();
 }
@@ -231,7 +231,7 @@
 };
 
 TEST(MessageQueueManager, ClearReentrant) {
-  std::unique_ptr<Thread> t(Thread::Create());
+  Thread t;
   EmptyHandler handler;
   RefCountedHandler* inner_handler(
       new rtc::RefCountedObject<RefCountedHandler>());
@@ -242,7 +242,7 @@
   // The inner handler will be removed in a re-entrant fashion from the
   // message queue of the thread while the outer handler is removed, verifying
   // that the iterator is not invalidated in "MessageQueue::Clear".
-  t->Post(RTC_FROM_HERE, inner_handler, 0);
-  t->Post(RTC_FROM_HERE, &handler, 0,
-          new ScopedRefMessageData<RefCountedHandler>(inner_handler));
+  t.Post(RTC_FROM_HERE, inner_handler, 0);
+  t.Post(RTC_FROM_HERE, &handler, 0,
+      new ScopedRefMessageData<RefCountedHandler>(inner_handler));
 }