Remove const from ThreadChecker in NullAudioPoller.
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 <efeae16329b3cb4ac5daf80d4bfda3344c027db3@webrtc.org>
Reviewed-by: Björn Terelius <efeae16329b3cb4ac5daf80d4bfda3344c027db3@webrtc.org>
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#20505}
TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc