Revert "Remove const from ThreadChecker in NullAudioPoller."

This reverts commit 54e41dd08a06f978a6d123f19f5bbe39370f04ea.

Reason for revert: We are reverting also https://webrtc-review.googlesource.com/c/src/+/16180, so this CL will be included in the re-land of https://webrtc-review.googlesource.com/c/src/+/16180.

Original change's description:
> Remove const from ThreadChecker in NullAudioPoller.
> 
> TBR=henrika@webrtc.org,solenberg@webrtc.org
> 
> Bug: webrtc:8482
> Change-Id: Ib2738224e776618c692db95cd9473335bc17be15
> Reviewed-on: https://webrtc-review.googlesource.com/17540
> Commit-Queue: Björn Terelius <terelius@webrtc.org>
> Reviewed-by: Björn Terelius <terelius@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#20505}

TBR=terelius@webrtc.org

Change-Id: I27c70ce331043ffdfec676c7e1a51e741d2fe770
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: webrtc:8482
Reviewed-on: https://webrtc-review.googlesource.com/17700
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20511}
diff --git a/audio/null_audio_poller.h b/audio/null_audio_poller.h
index b6ddf17..27c7e99 100644
--- a/audio/null_audio_poller.h
+++ b/audio/null_audio_poller.h
@@ -27,7 +27,7 @@
   void OnMessage(rtc::Message* msg) override;
 
  private:
-  rtc::ThreadChecker thread_checker_;
+  const rtc::ThreadChecker thread_checker_;
   AudioTransport* const audio_transport_;
   int64_t reschedule_at_;
 };