Reland "Replace RTC_WARN_UNUSED_RESULT with ABSL_MUST_USE_RESULT in c++ code"

This is a reland of 8c2250eddc7263036397179a0794b9b17d7afb38

Original change's description:
> Replace RTC_WARN_UNUSED_RESULT with ABSL_MUST_USE_RESULT in c++ code
>
> Bug: webrtc:12336
> Change-Id: If76f00d0883b5c8a90d3ef5554f5e22384b3fb58
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/197620
> Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
> Reviewed-by: Christoffer Rodbro <crodbro@webrtc.org>
> Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#32978}

Bug: webrtc:12336
Change-Id: I1cd017d45c1578528dec4532345950e9823f4a63
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/201732
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Christoffer Rodbro <crodbro@webrtc.org>
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33003}
diff --git a/rtc_base/synchronization/mutex.h b/rtc_base/synchronization/mutex.h
index 620fe74..0023d90e 100644
--- a/rtc_base/synchronization/mutex.h
+++ b/rtc_base/synchronization/mutex.h
@@ -13,9 +13,9 @@
 
 #include <atomic>
 
+#include "absl/base/attributes.h"
 #include "absl/base/const_init.h"
 #include "rtc_base/checks.h"
-#include "rtc_base/system/unused.h"
 #include "rtc_base/thread_annotations.h"
 
 #if defined(WEBRTC_ABSL_MUTEX)
@@ -41,7 +41,7 @@
   void Lock() RTC_EXCLUSIVE_LOCK_FUNCTION() {
     impl_.Lock();
   }
-  RTC_WARN_UNUSED_RESULT bool TryLock() RTC_EXCLUSIVE_TRYLOCK_FUNCTION(true) {
+  ABSL_MUST_USE_RESULT bool TryLock() RTC_EXCLUSIVE_TRYLOCK_FUNCTION(true) {
     return impl_.TryLock();
   }
   void Unlock() RTC_UNLOCK_FUNCTION() {