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}

modules/video_coding/generic_decoder.h

diff --git a/modules/video_coding/generic_decoder.h b/modules/video_coding/generic_decoder.h
index 40fe667..b89d3f4 100644
--- a/modules/video_coding/generic_decoder.h
+++ b/modules/video_coding/generic_decoder.h
@@ -19,8 +19,8 @@
 #include "modules/video_coding/include/video_codec_interface.h"
 #include "modules/video_coding/timestamp_map.h"
 #include "modules/video_coding/timing.h"
-#include "rtc_base/critical_section.h"
 #include "rtc_base/experiments/field_trial_parser.h"
+#include "rtc_base/synchronization/mutex.h"
 #include "rtc_base/thread_checker.h"
 
 namespace webrtc {
@@ -70,7 +70,7 @@
   // from the same thread, and therfore a lock is not required to access it.
   VCMReceiveCallback* _receiveCallback = nullptr;
   VCMTiming* _timing;
-  rtc::CriticalSection lock_;
+  Mutex lock_;
   VCMTimestampMap _timestampMap RTC_GUARDED_BY(lock_);
   int64_t ntp_offset_;
   // Set by the field trial WebRTC-SlowDownDecoder to simulate a slow decoder.