Add support for user defined "pool" to GN.

tools/gn/builder_record.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_BUILDER_RECORD_H_
 #define TOOLS_GN_BUILDER_RECORD_H_
 
 #include <memory>
 #include <set>
 #include <utility>
@@ skipping 17 matching lines @@
 // You can also have null item pointers when the target is not required for
 // the current build (should_generate is false).
 class BuilderRecord {
  public:
   typedef std::set<BuilderRecord*> BuilderRecordSet;
 
   enum ItemType {
     ITEM_UNKNOWN,
     ITEM_TARGET,
     ITEM_CONFIG,
-    ITEM_TOOLCHAIN
+    ITEM_TOOLCHAIN,
+    ITEM_POOL
   };
 
   BuilderRecord(ItemType type, const Label& label);
   ~BuilderRecord();
 
   ItemType type() const { return type_; }
   const Label& label() const { return label_; }
 
   // Returns a user-ready name for the given type. e.g. "target".
   static const char* GetNameForType(ItemType type);
@@ skipping 54 matching lines @@
   bool resolved_;
 
   BuilderRecordSet all_deps_;
   BuilderRecordSet unresolved_deps_;
   BuilderRecordSet waiting_on_resolution_;
 
   DISALLOW_COPY_AND_ASSIGN(BuilderRecord);
 };
 
 #endif  // TOOLS_GN_BUILDER_RECORD_H_