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}

api/sequence_checker_unittest.cc

diff --git a/api/sequence_checker_unittest.cc b/api/sequence_checker_unittest.cc
index 4029b8c..21a0894 100644
--- a/api/sequence_checker_unittest.cc
+++ b/api/sequence_checker_unittest.cc
@@ -40,21 +40,14 @@
 };
 
 void RunOnDifferentThread(rtc::FunctionView<void()> run) {
-  struct Object {
-    static void Run(void* obj) {
-      auto* me = static_cast<Object*>(obj);
-      me->run();
-      me->thread_has_run_event.Set();
-    }
-
-    rtc::FunctionView<void()> run;
-    rtc::Event thread_has_run_event;
-  } object{run};
-
-  rtc::PlatformThread thread(&Object::Run, &object, "thread");
-  thread.Start();
-  EXPECT_TRUE(object.thread_has_run_event.Wait(1000));
-  thread.Stop();
+  rtc::Event thread_has_run_event;
+  rtc::PlatformThread::SpawnJoinable(
+      [&] {
+        run();
+        thread_has_run_event.Set();
+      },
+      "thread");
+  EXPECT_TRUE(thread_has_run_event.Wait(1000));
 }
 
 }  // namespace