Add support for user defined "pool" to GN.

tools/gn/item.h

 // Copyright (c) 2013 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_ITEM_H_
 #define TOOLS_GN_ITEM_H_
 
 #include <string>
 
 #include "tools/gn/label.h"
 #include "tools/gn/visibility.h"
 
 class Config;
 class ParseNode;
+class Pool;
 class Settings;
 class Target;
 class Toolchain;
 
 // A named item (target, config, etc.) that participates in the dependency
 // graph.
 class Item {
  public:
   Item(const Settings* settings, const Label& label);
   virtual ~Item();
 
   const Settings* settings() const { return settings_; }
 
   // This is guaranteed to never change after construction so this can be
   // accessed from any thread with no locking once the item is constructed.
   const Label& label() const { return label_; }
 
   const ParseNode* defined_from() const { return defined_from_; }
   void set_defined_from(const ParseNode* df) { defined_from_ = df; }
 
   Visibility& visibility() { return visibility_; }
   const Visibility& visibility() const { return visibility_; }
 
   // Manual RTTI.
   virtual Config* AsConfig();
   virtual const Config* AsConfig() const;
+  virtual Pool* AsPool();
+  virtual const Pool* AsPool() const;
   virtual Target* AsTarget();
   virtual const Target* AsTarget() const;
   virtual Toolchain* AsToolchain();
   virtual const Toolchain* AsToolchain() const;
 
   // Returns a name like "target" or "config" for the type of item this is, to
   // be used in logging and error messages.
   std::string GetItemTypeName() const;
 
   // Called when this item is resolved, meaning it and all of its dependents
   // have no unresolved deps. Returns true on success. Sets the error and
   // returns false on failure.
   virtual bool OnResolved(Err* err);
 
  private:
   const Settings* settings_;
   Label label_;
   const ParseNode* defined_from_;
 
   Visibility visibility_;
 };
 
 #endif  // TOOLS_GN_ITEM_H_