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/audio_coding/neteq/neteq_impl.h b/webrtc/modules/audio_coding/neteq/neteq_impl.h
index 817b697..02adcd3 100644
--- a/webrtc/modules/audio_coding/neteq/neteq_impl.h
+++ b/webrtc/modules/audio_coding/neteq/neteq_impl.h
@@ -338,7 +338,7 @@
   // Creates DecisionLogic object with the mode given by |playout_mode_|.
   virtual void CreateDecisionLogic() EXCLUSIVE_LOCKS_REQUIRED(crit_sect_);
 
-  mutable rtc::CriticalSection crit_sect_;
+  rtc::CriticalSection crit_sect_;
   const rtc::scoped_ptr<BufferLevelFilter> buffer_level_filter_
       GUARDED_BY(crit_sect_);
   const rtc::scoped_ptr<DecoderDatabase> decoder_database_