Add missing (and remove superfluous) 'explicit' from constructors.

base/thread_task_runner_handle.h

 // Copyright (c) 2012 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_THREAD_TASK_RUNNER_HANDLE_H_
 #define BASE_THREAD_TASK_RUNNER_HANDLE_H_
 
 #include "base/base_export.h"
 #include "base/memory/ref_counted.h"
 
 namespace base {
 
 class SingleThreadTaskRunner;
 
 // ThreadTaskRunnerHandle stores a reference to the main task runner
 // for each thread. Not more than one of these objects can be created
 // per thread. After an instance of this object is created the Get()
 // function will return the |task_runner| specified in the
 // constructor.
 class BASE_EXPORT ThreadTaskRunnerHandle {
  public:
   // Gets the SingleThreadTaskRunner for the current thread.
   static scoped_refptr<SingleThreadTaskRunner> Get();
 
-  ThreadTaskRunnerHandle(
+  explicit ThreadTaskRunnerHandle(
       const scoped_refptr<SingleThreadTaskRunner>& task_runner);
   ~ThreadTaskRunnerHandle();
 
  private:
   scoped_refptr<SingleThreadTaskRunner> task_runner_;
 };
 
 }  // namespace base
 
 #endif  // BASE_THREAD_TASK_RUNNER_HANDLE_H_