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}
diff --git a/webrtc/modules/bitrate_controller/bitrate_controller_impl.h b/webrtc/modules/bitrate_controller/bitrate_controller_impl.h
index d7888cc..74f3c14 100644
--- a/webrtc/modules/bitrate_controller/bitrate_controller_impl.h
+++ b/webrtc/modules/bitrate_controller/bitrate_controller_impl.h
@@ -74,7 +74,7 @@
   BitrateObserver* observer_;
   int64_t last_bitrate_update_ms_;
 
-  mutable rtc::CriticalSection critsect_;
+  rtc::CriticalSection critsect_;
   SendSideBandwidthEstimation bandwidth_estimation_ GUARDED_BY(critsect_);
   uint32_t reserved_bitrate_bps_ GUARDED_BY(critsect_);