Reland "Refactor the PlatformThread API."

This reverts commit 793bac569fdf1be16cbf24d7871d20d00bbec81b.

Reason for revert: rare compilation error fixed

Original change's description:
> Revert "Refactor the PlatformThread API."
>
> This reverts commit c89fdd716c4c8af608017c76f75bf27e4c3d602e.
>
> Reason for revert: Causes rare compilation error on win-libfuzzer-asan trybot.
> See https://ci.chromium.org/p/chromium/builders/try/win-libfuzzer-asan-rel/713745?
>
> Original change's description:
> > 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}
>
> # Not skipping CQ checks because original CL landed > 1 day ago.
>
> TBR=handellm@webrtc.org
>
> Bug: webrtc:12727
> Change-Id: Ic0146be8866f6dd3ad9c364fb8646650b8e07419
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/217583
> Reviewed-by: Guido Urdaneta <guidou@webrtc.org>
> Reviewed-by: Markus Handell <handellm@webrtc.org>
> Commit-Queue: Guido Urdaneta <guidou@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#33936}

# Not skipping CQ checks because this is a reland.

Bug: webrtc:12727
Change-Id: Ifd6f44eac72fed84474277a1be03eb84d2f4376e
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/217881
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Markus Handell <handellm@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33950}

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