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}

rtc_base/task_queue_stdlib.cc

@@ -36,11 +36,14 @@ rtc::ThreadPriority TaskQueuePriorityToThreadPriority(
     TaskQueueFactory::Priority priority) {
   switch (priority) {
     case TaskQueueFactory::Priority::HIGH:
-      return rtc::ThreadPriority::kRealtime;
+      return rtc::kRealtimePriority;
     case TaskQueueFactory::Priority::LOW:
-      return rtc::ThreadPriority::kLow;
+      return rtc::kLowPriority;
     case TaskQueueFactory::Priority::NORMAL:
-      return rtc::ThreadPriority::kNormal;
+      return rtc::kNormalPriority;
+    default:
+      RTC_NOTREACHED();
+      return rtc::kNormalPriority;
   }
 }
 
@@ -75,6 +78,8 @@ class TaskQueueStdlib final : public TaskQueueBase {
 
   NextTask GetNextTask();
 
+  static void ThreadMain(void* context);
+
   void ProcessTasks();
 
   void NotifyWake();
@@ -121,13 +126,11 @@ TaskQueueStdlib::TaskQueueStdlib(absl::string_view queue_name,
     : started_(/*manual_reset=*/false, /*initially_signaled=*/false),
       stopped_(/*manual_reset=*/false, /*initially_signaled=*/false),
       flag_notify_(/*manual_reset=*/false, /*initially_signaled=*/false),
-      thread_(rtc::PlatformThread::SpawnJoinable(
-          [this] {
-            CurrentTaskQueueSetter set_current(this);
-            ProcessTasks();
-          },
-          queue_name,
-          rtc::ThreadAttributes().SetPriority(priority))) {
+      thread_(&TaskQueueStdlib::ThreadMain,
+              this,
+              queue_name,
+              rtc::ThreadAttributes().SetPriority(priority)) {
+  thread_.Start();
   started_.Wait(rtc::Event::kForever);
 }
 
@@ -142,7 +145,7 @@ void TaskQueueStdlib::Delete() {
   NotifyWake();
 
   stopped_.Wait(rtc::Event::kForever);
-  thread_.Finalize();
+  thread_.Stop();
   delete this;
 }
 
@@ -219,6 +222,13 @@ TaskQueueStdlib::NextTask TaskQueueStdlib::GetNextTask() {
   return result;
 }
 
+// static
+void TaskQueueStdlib::ThreadMain(void* context) {
+  TaskQueueStdlib* me = static_cast<TaskQueueStdlib*>(context);
+  CurrentTaskQueueSetter set_current(me);
+  me->ProcessTasks();
+}
+
 void TaskQueueStdlib::ProcessTasks() {
   started_.Set();