Add support for user defined "pool" to GN.

tools/gn/BUILD.gn

 # 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.
 
 import("//testing/test.gni")
 
 defines = [ "GN_BUILD" ]
 
 static_library("gn_lib") {
   configs += [ "//build/config:precompiled_headers" ]
@@ skipping 128 matching lines @@
     "output_file.cc",
     "output_file.h",
     "parse_tree.cc",
     "parse_tree.h",
     "parser.cc",
     "parser.h",
     "path_output.cc",
     "path_output.h",
     "pattern.cc",
     "pattern.h",
+    "pool.cc",
+    "pool.h",
     "qt_creator_writer.cc",
     "qt_creator_writer.h",
     "runtime_deps.cc",
     "runtime_deps.h",
     "scheduler.cc",
     "scheduler.h",
     "scope.cc",
     "scope.h",
     "scope_per_file_provider.cc",
     "scope_per_file_provider.h",
@@ skipping 169 matching lines @@
     "format_test_data/",
   ]
 
   deps = [
     ":gn_lib",
     "//base/test:run_all_unittests",
     "//base/test:test_support",
     "//testing/gtest",
   ]
 }