Make the constructor of TaskSchedulerImpl explicit.

base/task_scheduler/task_scheduler_impl.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef BASE_TASK_SCHEDULER_TASK_SCHEDULER_IMPL_H_
 #define BASE_TASK_SCHEDULER_TASK_SCHEDULER_IMPL_H_
 
 #include <stddef.h>
 
 #include <memory>
@@ skipping 64 matching lines @@
   scoped_refptr<TaskRunner> CreateTaskRunnerWithTraits(
       const TaskTraits& traits,
       ExecutionMode execution_mode) override;
   void Shutdown() override;
 
   // Joins all threads of this scheduler. Tasks that are already running are
   // allowed to complete their execution. This can only be called once.
   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);
 
   // Returns the worker pool that runs Tasks with |traits|.
   SchedulerWorkerPool* GetWorkerPoolForTraits(const TaskTraits& traits);
 
   // Callback invoked when a non-single-thread |sequence| isn't empty after a
   // worker pops a Task from it.
   void ReEnqueueSequenceCallback(scoped_refptr<Sequence> sequence);
 
@@ skipping 12 matching lines @@
   WaitableEvent join_for_testing_returned_;
 #endif
 
   DISALLOW_COPY_AND_ASSIGN(TaskSchedulerImpl);
 };
 
 }  // namespace internal
 }  // namespace base
 
 #endif  // BASE_TASK_SCHEDULER_TASK_SCHEDULER_IMPL_H_