Added memory pressure monitor for linux.

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::g_monitor = nullptr;
 
 // static
 MemoryPressureMonitor* MemoryPressureMonitor::Get() {
   return g_monitor;
 }
 
 }  // namespace base