Optional <TimeDelta> since they may be non existent.

base/base.gyp

 # Copyright (c) 2012 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.
 
 {
   'variables': {
     'chromium_code': 1,
   },
   'includes': [
     '../build/win_precompile.gypi',
@@ skipping 481 matching lines @@
         'metrics/metrics_hashes_unittest.cc',
         'metrics/persistent_histogram_allocator_unittest.cc',
         'metrics/persistent_memory_allocator_unittest.cc',
         'metrics/persistent_sample_map_unittest.cc',
         'metrics/sample_map_unittest.cc',
         'metrics/sample_vector_unittest.cc',
         'metrics/sparse_histogram_unittest.cc',
         'metrics/statistics_recorder_unittest.cc',
         'native_library_unittest.cc',
         'numerics/safe_numerics_unittest.cc',
         'observer_list_unittest.cc',

[shivanisha, 2016/03/29 18:57:16]

Ignore this file for review. Added here for compilation, part of 
https://codereview.chromium.org/1245163002/

+        'optional_unittest.cc',
         'os_compat_android_unittest.cc',
         'path_service_unittest.cc',
         'pickle_unittest.cc',
         'posix/file_descriptor_shuffle_unittest.cc',
         'posix/unix_domain_socket_linux_unittest.cc',
         'power_monitor/power_monitor_unittest.cc',
         'process/memory_unittest.cc',
         'process/memory_unittest_mac.h',
         'process/memory_unittest_mac.mm',
         'process/process_metrics_unittest.cc',
@@ skipping 1201 matching lines @@
             '../build/isolate.gypi',
           ],
           'sources': [
             'base_unittests.isolate',
           ],
         },
       ],
     }],
   ],
 }