@Override
  public synchronized void execute(Runnable runnable) {
    super.execute(runnable);
  }
Synchronized override of {@link ConstrainedExecutorService#execute(Runnable)} to ensure that
view of memory is consistent between different threads executing tasks serially.

@param runnable The task to be executed.
fbcore/src/main/java/com/facebook/common/executors/DefaultSerialExecutorService.java