Fix -Wunused-but-set-variable.

Bug: None
Change-Id: I3be977ac0536cd6686f73a9e51b7f8adff842d31
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/236480
Reviewed-by: Markus Handell <handellm@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#35268}

rtc_base/synchronization/mutex_unittest.cc

@@ -103,7 +103,7 @@ class LockRunner : public rtc::MessageHandlerAutoCleanup {
     locker_.Lock();
     shared_value = shared_value_;
     locker_.Unlock();
-    return shared_value_;
+    return shared_value;
   }
 
   void OnMessage(Message* msg) override {