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/video_coding/generic_encoder.h b/webrtc/modules/video_coding/generic_encoder.h
index f739edb..da7297f 100644
--- a/webrtc/modules/video_coding/generic_encoder.h
+++ b/webrtc/modules/video_coding/generic_encoder.h
@@ -138,7 +138,7 @@
   VideoEncoderRateObserver* const rate_observer_;
   VCMEncodedFrameCallback* const vcm_encoded_frame_callback_;
   const bool internal_source_;
-  mutable rtc::CriticalSection params_lock_;
+  rtc::CriticalSection params_lock_;
   EncoderParameters encoder_params_ GUARDED_BY(params_lock_);
   VideoRotation rotation_;
   bool is_screenshare_;