[Mac] Maximise timer slack for background tabs

base/message_loop/timer_slack.cc

+// Copyright 2014 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.
+
+#include "base/message_loop/timer_slack.h"
+
+namespace {
+
+base::TimerSlack* TimerSlackStorage() {
+  static base::TimerSlack timer_slack = base::TIMER_SLACK_NONE;
+  return &timer_slack;
+}
+
+}  // namespace
+
+namespace base {
+
+void SetTimerSlack(TimerSlack timer_slack) {
+  *TimerSlackStorage() = timer_slack;
+}
+
+TimerSlack GetTimerSlack() {
+  return *TimerSlackStorage();
+}
+
+}  // namespace base