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

This is a partial re-land. The change doesn't make the default Thread ctor protected anymore but it does mark it as deprecated and updates all use of it in WebRTC.

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/2977953002
Cr-Commit-Position: refs/heads/master@{#19031}

webrtc/rtc_base/timeutils_unittest.cc

diff --git a/webrtc/rtc_base/timeutils_unittest.cc b/webrtc/rtc_base/timeutils_unittest.cc
index 5fd9436..a409fb6 100644
--- a/webrtc/rtc_base/timeutils_unittest.cc
+++ b/webrtc/rtc_base/timeutils_unittest.cc
@@ -349,8 +349,8 @@
   FakeClock clock;
   SetClockForTesting(&clock);
 
-  Thread worker;
-  worker.Start();
+  std::unique_ptr<Thread> worker(Thread::CreateWithSocketServer());
+  worker->Start();
 
   // Post an event that won't be executed for 10 seconds.
   Event message_handler_dispatched(false, false);
@@ -358,7 +358,7 @@
     message_handler_dispatched.Set();
   };
   FunctorMessageHandler<void, decltype(functor)> handler(functor);
-  worker.PostDelayed(RTC_FROM_HERE, 60000, &handler);
+  worker->PostDelayed(RTC_FROM_HERE, 60000, &handler);
 
   // Wait for a bit for the worker thread to be started and enter its socket
   // select(). Otherwise this test would be trivial since the worker thread
@@ -369,7 +369,7 @@
   // and dispatch the message instantly.
   clock.AdvanceTime(TimeDelta::FromSeconds(60u));
   EXPECT_TRUE(message_handler_dispatched.Wait(0));
-  worker.Stop();
+  worker->Stop();
 
   SetClockForTesting(nullptr);