Add support for user defined "pool" to GN.

tools/gn/toolchain.cc

 // 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.
 
 #include "tools/gn/toolchain.h"
 
 #include <stddef.h>
 #include <string.h>
 #include <utility>
 
@@ skipping 68 matching lines @@
     case TYPE_STAMP: return kToolStamp;
     case TYPE_COPY: return kToolCopy;
     case TYPE_COPY_BUNDLE_DATA: return kToolCopyBundleData;
     case TYPE_COMPILE_XCASSETS: return kToolCompileXCAssets;
     default:
       NOTREACHED();
       return std::string();
   }
 }
 
+Tool* Toolchain::GetTool(ToolType type) {
+  DCHECK(type != TYPE_NONE);
+  return tools_[static_cast<size_t>(type)].get();
+}
+
 const Tool* Toolchain::GetTool(ToolType type) const {
   DCHECK(type != TYPE_NONE);
   return tools_[static_cast<size_t>(type)].get();
 }
 
 void Toolchain::SetTool(ToolType type, std::unique_ptr<Tool> t) {
   DCHECK(type != TYPE_NONE);
   DCHECK(!tools_[type].get());
   t->SetComplete();
   tools_[type] = std::move(t);
@@ skipping 66 matching lines @@
       return TYPE_STAMP;
     default:
       NOTREACHED();
       return Toolchain::TYPE_NONE;
   }
 }
 
 const Tool* Toolchain::GetToolForTargetFinalOutput(const Target* target) const {
   return tools_[GetToolTypeForTargetFinalOutput(target)].get();
 }