Optional <TimeDelta> since they may be non existent.

base/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.
 
 # HOW TO WRITE CONDITIONALS IN THIS FILE
 # ======================================
 #
 # In many other places, one would write a conditional that expresses all the
 # cases when a source file is used or unused, and then either add or subtract
 # it from the sources list in that case
@@ skipping 546 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 1186 matching lines @@
     "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",
+    "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 536 matching lines @@
 
   # GYP: //base.gyp:base_java_unittest_support
   android_library("base_java_unittest_support") {
     deps = [
       ":base_java",
     ]
     java_files =
         [ "test/android/java/src/org/chromium/base/ContentUriTestUtils.java" ]
   }
 }