Make the constructor of TaskSchedulerImpl explicit.

base/task_scheduler/task_scheduler_impl.h

Index: base/task_scheduler/task_scheduler_impl.h
diff --git a/base/task_scheduler/task_scheduler_impl.h b/base/task_scheduler/task_scheduler_impl.h
index 02cbe3c6c51b29a0b08f89a9aa3f450ac0a128dc..ac827f7de53b0c1655f76b7944e885947a0ce62c 100644
--- a/base/task_scheduler/task_scheduler_impl.h
+++ b/base/task_scheduler/task_scheduler_impl.h
@@ -82,8 +82,8 @@ class BASE_EXPORT TaskSchedulerImpl : public TaskScheduler {
   void JoinForTesting();
 
  private:
-  TaskSchedulerImpl(const WorkerPoolIndexForTraitsCallback&
-                        worker_pool_index_for_traits_callback);
+  explicit TaskSchedulerImpl(const WorkerPoolIndexForTraitsCallback&
+                                 worker_pool_index_for_traits_callback);
 
   void Initialize(const std::vector<WorkerPoolCreationArgs>& worker_pools);