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}

modules/audio_device/linux/audio_device_pulse_linux.h

@@ -268,8 +268,9 @@ class AudioDeviceLinuxPulse : public AudioDeviceGeneric {
   rtc::Event _recStartEvent;
   rtc::Event _playStartEvent;
 
-  rtc::PlatformThread _ptrThreadPlay;
-  rtc::PlatformThread _ptrThreadRec;
+  // TODO(pbos): Remove unique_ptr and use directly without resetting.
+  std::unique_ptr<rtc::PlatformThread> _ptrThreadPlay;
+  std::unique_ptr<rtc::PlatformThread> _ptrThreadRec;
 
   AudioMixerManagerLinuxPulse _mixerManager;