Add support for user defined "pool" to GN.

tools/gn/toolchain.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_TOOLCHAIN_H_
 #define TOOLS_GN_TOOLCHAIN_H_
 
 #include <memory>
 
 #include "base/logging.h"
@@ skipping 60 matching lines @@
 
   // Item overrides.
   Toolchain* AsToolchain() override;
   const Toolchain* AsToolchain() const override;
 
   // Returns TYPE_NONE on failure.
   static ToolType ToolNameToType(const base::StringPiece& str);
   static std::string ToolTypeToName(ToolType type);
 
   // Returns null if the tool hasn't been defined.
+  Tool* GetTool(ToolType type);
   const Tool* GetTool(ToolType type) const;
 
   // Set a tool. When all tools are configured, you should call
   // ToolchainSetupComplete().
   void SetTool(ToolType type, std::unique_ptr<Tool> t);
 
   // Does final setup on the toolchain once all tools are known.
   void ToolchainSetupComplete();
 
   // Targets that must be resolved before compiling any targets.
@@ skipping 36 matching lines @@
   bool setup_complete_;
 
   // Substitutions used by the tools in this toolchain.
   SubstitutionBits substitution_bits_;
 
   LabelTargetVector deps_;
   Scope::KeyValueMap args_;
 };
 
 #endif  // TOOLS_GN_TOOLCHAIN_H_