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}
diff --git a/audio/null_audio_poller.h b/audio/null_audio_poller.h
index 27c7e99..b6ddf17 100644
--- a/audio/null_audio_poller.h
+++ b/audio/null_audio_poller.h
@@ -27,7 +27,7 @@
   void OnMessage(rtc::Message* msg) override;
 
  private:
-  const rtc::ThreadChecker thread_checker_;
+  rtc::ThreadChecker thread_checker_;
   AudioTransport* const audio_transport_;
   int64_t reschedule_at_;
 };