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_processing/audio_processing_impl.h b/webrtc/modules/audio_processing/audio_processing_impl.h
index fbb9b6e..5da3996 100644
--- a/webrtc/modules/audio_processing/audio_processing_impl.h
+++ b/webrtc/modules/audio_processing/audio_processing_impl.h
@@ -239,15 +239,15 @@
       EXCLUSIVE_LOCKS_REQUIRED(crit_capture_);
 
   // Critical section.
-  mutable rtc::CriticalSection crit_debug_;
+  rtc::CriticalSection crit_debug_;
 
   // Debug dump state.
   ApmDebugDumpState debug_dump_;
 #endif
 
   // Critical sections.
-  mutable rtc::CriticalSection crit_render_ ACQUIRED_BEFORE(crit_capture_);
-  mutable rtc::CriticalSection crit_capture_;
+  rtc::CriticalSection crit_render_ ACQUIRED_BEFORE(crit_capture_);
+  rtc::CriticalSection crit_capture_;
 
   // Structs containing the pointers to the submodules.
   rtc::scoped_ptr<ApmPublicSubmodules> public_submodules_;