Add support for user defined "pool" to GN.

tools/gn/gn.gyp

 {
   'variables': {
     'chromium_code': 1,
   },
   'targets': [
     {
       'target_name': 'gn_lib',
       'type': 'static_library',
       'dependencies': [
         '../../base/base.gyp:base',
@@ 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 148 matching lines @@
           'dependencies': [
             'gn_unittests',
           ],
           'includes': [ '../../build/isolate.gypi' ],
           'sources': [ 'gn_unittests.isolate' ],
         },
       ],
     }],
   ],
 }