Optional <TimeDelta> since they may be non existent.

base/base.gypi

 # 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.
 
 {
   'includes': [
     'trace_event/trace_event.gypi',
   ],
   'target_defaults': {
     'variables': {
@@ skipping 426 matching lines @@
           'native_library_win.cc',
           'nix/mime_util_xdg.cc',
           'nix/mime_util_xdg.h',
           'nix/xdg_util.cc',
           'nix/xdg_util.h',
           'numerics/safe_conversions.h',
           'numerics/safe_conversions_impl.h',
           'numerics/safe_math.h',
           'numerics/safe_math_impl.h',
           'observer_list.h',
           'observer_list_threadsafe.h',

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

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

+          'optional.h',
           'os_compat_android.cc',
           'os_compat_android.h',
           'os_compat_nacl.cc',
           'os_compat_nacl.h',
           'path_service.cc',
           'path_service.h',
           'pending_task.cc',
           'pending_task.h',
           'pickle.cc',
           'pickle.h',
@@ skipping 619 matching lines @@
           'i18n/time_formatting.h',
           'i18n/timezone.cc',
           'i18n/timezone.h',
           'i18n/utf8_validator_tables.cc',
           'i18n/utf8_validator_tables.h',
         ],
       }]
     ],
   },
 }