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_processing/video_processing_impl.h b/webrtc/modules/video_processing/video_processing_impl.h
index edbaba1..1d9a377 100644
--- a/webrtc/modules/video_processing/video_processing_impl.h
+++ b/webrtc/modules/video_processing/video_processing_impl.h
@@ -44,7 +44,7 @@
   VideoContentMetrics* GetContentMetrics() const override;
 
  private:
-  mutable rtc::CriticalSection mutex_;
+  rtc::CriticalSection mutex_;
   VPMDeflickering deflickering_ GUARDED_BY(mutex_);
   VPMBrightnessDetection brightness_detection_;
   VPMFramePreprocessor frame_pre_processor_;