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/call/call_perf_tests.cc

@@ -113,7 +113,7 @@ class SyncRtcpObserver : public test::RtpRtcpObserver {
     ntp_rtp_pairs_.push_front(ntp_rtp_pair);
   }
 
-  mutable rtc::CriticalSection crit_;
+  rtc::CriticalSection crit_;
   RtcpList ntp_rtp_pairs_ GUARDED_BY(crit_);
 };