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/call/bitrate_allocator.h b/webrtc/call/bitrate_allocator.h
index 88a9960..5028e12 100644
--- a/webrtc/call/bitrate_allocator.h
+++ b/webrtc/call/bitrate_allocator.h
@@ -89,7 +89,7 @@
   ObserverBitrateMap LowRateAllocation(uint32_t bitrate)
       EXCLUSIVE_LOCKS_REQUIRED(crit_sect_);
 
-  mutable rtc::CriticalSection crit_sect_;
+  rtc::CriticalSection crit_sect_;
   // Stored in a list to keep track of the insertion order.
   BitrateObserverConfList bitrate_observers_ GUARDED_BY(crit_sect_);
   bool bitrate_observers_modified_ GUARDED_BY(crit_sect_);