Allow inherited constructors and add a sample usage.

tools/gn/pool.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 TOOLS_GN_POOL_H_
 #define TOOLS_GN_POOL_H_
 
 #include <string>
 
 #include "tools/gn/item.h"
 
 // Represents a named pool in the dependency graph.
 //
 // A pool is used to limit the parallelism of task invocation in the
 // generated ninja build. Pools are referenced by toolchains.
 class Pool : public Item {
  public:
-  Pool(const Settings* settings, const Label& label);
+  using Item::Item;
   ~Pool() override;
 
   Pool(const Pool&) = delete;
   Pool& operator=(const Pool&) = delete;
 
   // Item implementation.
   Pool* AsPool() override;
   const Pool* AsPool() const override;
 
   // The pool depth (number of task to run simultaneously).
   int64_t depth() const { return depth_; }
   void set_depth(int64_t depth) { depth_ = depth; }
 
   // The pool name in generated ninja files.
   std::string GetNinjaName(const Label& default_toolchain) const;
 
  private:
   std::string GetNinjaName(bool include_toolchain) const;
 
   int64_t depth_ = 0;
 };
 
 #endif  // TOOLS_GN_POOL_H_