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

net/socket/client_socket_pool_histograms.h

 // Copyright (c) 2011 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 NET_SOCKET_CLIENT_SOCKET_POOL_HISTOGRAMS_H_
 #define NET_SOCKET_CLIENT_SOCKET_POOL_HISTOGRAMS_H_
 
 #include <string>
 
 #include "base/memory/ref_counted.h"
 #include "base/time.h"
 #include "net/base/net_export.h"
 
 namespace base {
 class Histogram;
 }
 
 namespace net {
 
 class NET_EXPORT_PRIVATE ClientSocketPoolHistograms {
  public:
-  ClientSocketPoolHistograms(const std::string& pool_name);
+  explicit ClientSocketPoolHistograms(const std::string& pool_name);
   ~ClientSocketPoolHistograms();
 
   void AddSocketType(int socket_reuse_type) const;
   void AddRequestTime(base::TimeDelta time) const;
   void AddUnusedIdleTime(base::TimeDelta time) const;
   void AddReusedIdleTime(base::TimeDelta time) const;
 
  private:
   base::Histogram* socket_type_;
   base::Histogram* request_time_;
   base::Histogram* unused_idle_time_;
   base::Histogram* reused_idle_time_;
 
   bool is_http_proxy_connection_;
   bool is_socks_connection_;
 
   DISALLOW_COPY_AND_ASSIGN(ClientSocketPoolHistograms);
 };
 
 }  // namespace net
 
 #endif  // NET_SOCKET_CLIENT_SOCKET_POOL_HISTOGRAMS_H_