Refactor the PlatformThread API.

PlatformThread's API is using old style function pointers, causes
casting, is unintuitive and forces artificial call sequences, and
is additionally possible to misuse in release mode.

Fix this by an API face lift:
1. The class is turned into a handle, which can be empty.
2. The only way of getting a non-empty PlatformThread is by calling
SpawnJoinable or SpawnDetached, clearly conveying the semantics to the
code reader.
3. Handles can be Finalized, which works differently for joinable and
detached threads:
  a) Handles for detached threads are simply closed where applicable.
  b) Joinable threads are joined before handles are closed.
4. The destructor finalizes handles. No explicit call is needed.

Fixed: webrtc:12727
Change-Id: Id00a0464edf4fc9e552b6a1fbb5d2e1280e88811
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/215075
Commit-Queue: Markus Handell <handellm@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Tommi <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33923}

rtc_base/task_queue_libevent.cc

@@ -93,16 +93,12 @@ void EventAssign(struct event* ev,
 rtc::ThreadPriority TaskQueuePriorityToThreadPriority(Priority priority) {
   switch (priority) {
     case Priority::HIGH:
-      return rtc::kRealtimePriority;
+      return rtc::ThreadPriority::kRealtime;
     case Priority::LOW:
-      return rtc::kLowPriority;
+      return rtc::ThreadPriority::kLow;
     case Priority::NORMAL:
-      return rtc::kNormalPriority;
-    default:
-      RTC_NOTREACHED();
-      break;
+      return rtc::ThreadPriority::kNormal;
   }
-  return rtc::kNormalPriority;
 }
 
 class TaskQueueLibevent final : public TaskQueueBase {
@@ -120,7 +116,6 @@ class TaskQueueLibevent final : public TaskQueueBase {
 
   ~TaskQueueLibevent() override = default;
 
-  static void ThreadMain(void* context);
   static void OnWakeup(int socket, short flags, void* context);  // NOLINT
   static void RunTimer(int fd, short flags, void* context);      // NOLINT
 
@@ -172,11 +167,7 @@ class TaskQueueLibevent::SetTimerTask : public QueuedTask {
 
 TaskQueueLibevent::TaskQueueLibevent(absl::string_view queue_name,
                                      rtc::ThreadPriority priority)
-    : event_base_(event_base_new()),
-      thread_(&TaskQueueLibevent::ThreadMain,
-              this,
-              queue_name,
-              rtc::ThreadAttributes().SetPriority(priority)) {
+    : event_base_(event_base_new()) {
   int fds[2];
   RTC_CHECK(pipe(fds) == 0);
   SetNonBlocking(fds[0]);
@@ -187,7 +178,18 @@ TaskQueueLibevent::TaskQueueLibevent(absl::string_view queue_name,
   EventAssign(&wakeup_event_, event_base_, wakeup_pipe_out_,
               EV_READ | EV_PERSIST, OnWakeup, this);
   event_add(&wakeup_event_, 0);
-  thread_.Start();
+  thread_ = rtc::PlatformThread::SpawnJoinable(
+      [this] {
+        {
+          CurrentTaskQueueSetter set_current(this);
+          while (is_active_)
+            event_base_loop(event_base_, 0);
+        }
+
+        for (TimerEvent* timer : pending_timers_)
+          delete timer;
+      },
+      queue_name, rtc::ThreadAttributes().SetPriority(priority));
 }
 
 void TaskQueueLibevent::Delete() {
@@ -202,7 +204,7 @@ void TaskQueueLibevent::Delete() {
     nanosleep(&ts, nullptr);
   }
 
-  thread_.Stop();
+  thread_.Finalize();
 
   event_del(&wakeup_event_);
 
@@ -255,20 +257,6 @@ void TaskQueueLibevent::PostDelayedTask(std::unique_ptr<QueuedTask> task,
   }
 }
 
-// static
-void TaskQueueLibevent::ThreadMain(void* context) {
-  TaskQueueLibevent* me = static_cast<TaskQueueLibevent*>(context);
-
-  {
-    CurrentTaskQueueSetter set_current(me);
-    while (me->is_active_)
-      event_base_loop(me->event_base_, 0);
-  }
-
-  for (TimerEvent* timer : me->pending_timers_)
-    delete timer;
-}
-
 // static
 void TaskQueueLibevent::OnWakeup(int socket,
                                  short flags,  // NOLINT