Allow inherited constructors and add a sample usage.

tools/gn/pool.cc

 // 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.
 
 #include "tools/gn/pool.h"
 
 #include <sstream>
 
 #include "base/logging.h"
 
-Pool::Pool(const Settings* settings, const Label& label)
-    : Item(settings, label) {}
-
 Pool::~Pool() {}
 
 Pool* Pool::AsPool() {
   return this;
 }
 
 const Pool* Pool::AsPool() const {
   return this;
 }
 
@@ skipping 15 matching lines @@
   }
 
   DCHECK(label().dir().is_source_absolute());
   std::string label_dir = label().dir().value();
   for (std::string::size_type i = 2; i < label_dir.size(); ++i) {
     buffer << (label_dir[i] == '/' ? '_' : label_dir[i]);
   }
   buffer << label().name();
   return buffer.str();
 }