Update from https://crrev.com/327068

base/memory/memory_pressure_monitor.cc

+// Copyright 2015 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/memory/memory_pressure_monitor.h"
+
+#include "base/logging.h"
+
+namespace base {
+namespace {
+
+MemoryPressureMonitor* g_monitor = nullptr;
+
+}  // namespace
+
+MemoryPressureMonitor::MemoryPressureMonitor() {
+  DCHECK(!g_monitor);
+  g_monitor = this;
+}
+
+MemoryPressureMonitor::~MemoryPressureMonitor() {
+  DCHECK(g_monitor);
+  g_monitor = nullptr;
+}
+
+// static
+MemoryPressureMonitor* MemoryPressureMonitor::Get() {
+  return g_monitor;
+}
+
+}  // namespace base