Migrate modules/video_coding to webrtc::Mutex.

Bug: webrtc:11567
Change-Id: I8023fbe7595f7ba8ae7c7db3583fc2e560ec3df2
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/178803
Commit-Queue: Markus Handell <handellm@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#31644}
diff --git a/modules/video_coding/video_coding_impl.h b/modules/video_coding/video_coding_impl.h
index eaab639..440d199 100644
--- a/modules/video_coding/video_coding_impl.h
+++ b/modules/video_coding/video_coding_impl.h
@@ -24,6 +24,7 @@
 #include "modules/video_coding/receiver.h"
 #include "modules/video_coding/timing.h"
 #include "rtc_base/one_time_event.h"
+#include "rtc_base/synchronization/mutex.h"
 #include "rtc_base/synchronization/sequence_checker.h"
 #include "rtc_base/thread_annotations.h"
 #include "rtc_base/thread_checker.h"
@@ -100,7 +101,7 @@
   rtc::ThreadChecker decoder_thread_checker_;
   rtc::ThreadChecker module_thread_checker_;
   Clock* const clock_;
-  rtc::CriticalSection process_crit_;
+  Mutex process_mutex_;
   VCMTiming* _timing;
   VCMReceiver _receiver;
   VCMDecodedFrameCallback _decodedFrameCallback;
@@ -111,8 +112,8 @@
   VCMPacketRequestCallback* _packetRequestCallback;
 
   // Used on both the module and decoder thread.
-  bool _scheduleKeyRequest RTC_GUARDED_BY(process_crit_);
-  bool drop_frames_until_keyframe_ RTC_GUARDED_BY(process_crit_);
+  bool _scheduleKeyRequest RTC_GUARDED_BY(process_mutex_);
+  bool drop_frames_until_keyframe_ RTC_GUARDED_BY(process_mutex_);
 
   // Modified on the construction thread while not attached to the process
   // thread.  Once attached to the process thread, its value is only read