message: Remove RTC_WARN_UNUSED_RESULT in favor of ABSL_MUST_USE_RESULT.
  This header landed recently but it was not built, so this wasn't
  caught.
  Cr-Commit-Position: refs/heads/master@{#33859}
diff: #include <atomic>
  +#include "absl/base/attributes.h"
  #include "rtc_base/checks.h"
  #include "rtc_base/system/unused.h"
  #include "rtc_base/thread_annotations.h"
  @@ -41,7 +42,7 @@ class RTC_LOCKABLE MutexImpl final {
  RTC_CHECK(was_free)
  << "WEBRTC_RACE_CHECK_MUTEX: mutex locked concurrently.";
  }
  - RTC_WARN_UNUSED_RESULT bool TryLock() RTC_EXCLUSIVE_TRYLOCK_FUNCTION(true) {
  + ABSL_MUST_USE_RESULT bool TryLock() RTC_EXCLUSIVE_TRYLOCK_FUNCTION(true) {
  bool was_free = free_.exchange(false, std::memory_order_acquire);
  return was_free;
  }
  