Remove mutable from rtc::CriticalSection members.

rtc::CriticalSection is now lockable from const methods and no longer
need to remain mutable.

BUG=
R=tommi@webrtc.org

Review URL: https://codereview.webrtc.org/1613643004

Cr-Commit-Position: refs/heads/master@{#11367}

webrtc/modules/audio_coding/codecs/isac/locked_bandwidth_info.h

@@ -36,7 +36,7 @@ class LockedIsacBandwidthInfo final {
   }
 
  private:
-  mutable rtc::CriticalSection lock_;
+  rtc::CriticalSection lock_;
   IsacBandwidthInfo bwinfo_ GUARDED_BY(lock_);
 };