[Mac] Maximise timer slack for background tabs

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.
 
 {
   'target_defaults': {
     'variables': {
       'base_target': 0,
       'base_i18n_target': 0,
     },
@@ skipping 339 matching lines @@
           'message_loop/message_loop_proxy_impl.cc',
           'message_loop/message_loop_proxy_impl.h',
           'message_loop/message_pump.cc',
           'message_loop/message_pump.h',
           'message_loop/message_pump_android.cc',
           'message_loop/message_pump_android.h',
           'message_loop/message_pump_default.cc',
           'message_loop/message_pump_default.h',
           'message_loop/message_pump_win.cc',
           'message_loop/message_pump_win.h',
+          'message_loop/timer_slack.h',
           'metrics/sample_map.cc',
           'metrics/sample_map.h',
           'metrics/sample_vector.cc',
           'metrics/sample_vector.h',
           'metrics/bucket_ranges.cc',
           'metrics/bucket_ranges.h',
           'metrics/histogram.cc',
           'metrics/histogram.h',
           'metrics/histogram_base.cc',
           'metrics/histogram_base.h',
@@ skipping 597 matching lines @@
           'i18n/time_formatting.h',
           'i18n/timezone.cc',
           'i18n/timezone.h',
           'i18n/utf8_validator_tables.cc',
           'i18n/utf8_validator_tables.h',
         ],
       }]
     ],
   },
 }