Add support for user defined "pool" to GN.

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 "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);
+  ~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; }
+
+ private:
+  int64_t depth_ = 0;
+};
+
+#endif  // TOOLS_GN_POOL_H_