Userland: Use Threading::MutexLocker to lock/unlock mutexes

Author: https://github.com/idispatch 🔰
Commit: 0c27d95e76
Pull-request: https://github.com/SerenityOS/serenity/pull/15854
Reviewed-by: https://github.com/awesomekling

Userland/Applications/PixelPaint/ImageProcessor.cpp

@@ -59,10 +59,8 @@ ErrorOr<void> ImageProcessor::enqueue_command(NonnullRefPtr<ImageProcessingComma
         m_processor_thread->detach();
     }
 
-    m_wakeup_mutex.lock();
+    Threading::MutexLocker const locker(m_wakeup_mutex);
     m_wakeup_variable.signal();
-    m_wakeup_mutex.unlock();
-
     return {};
 }