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-Original-Commit-Position: refs/heads/master@{#11367}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 5ad935cb56e8647d42dffdbb8897f31294e1c272
diff --git a/test/fake_audio_device.h b/test/fake_audio_device.h
index 7ca657b..ce480c1 100644
--- a/test/fake_audio_device.h
+++ b/test/fake_audio_device.h
@@ -58,7 +58,7 @@
 
   Clock* clock_;
   rtc::scoped_ptr<EventTimerWrapper> tick_;
-  mutable rtc::CriticalSection lock_;
+  rtc::CriticalSection lock_;
   rtc::PlatformThread thread_;
   rtc::scoped_ptr<ModuleFileUtility> file_utility_;
   rtc::scoped_ptr<FileWrapper> input_stream_;