Enable UMA histogram logger for low memor kill events in ArcMetricsService.

chrome/browser/chromeos/arc/arc_low_memory_killer_monitor.h

-// Copyright 2016 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.
-
-#ifndef CHROME_BROWSER_CHROMEOS_ARC_ARC_LOW_MEMORY_KILLER_MONITOR_H_
-#define CHROME_BROWSER_CHROMEOS_ARC_ARC_LOW_MEMORY_KILLER_MONITOR_H_
-
-#include "base/macros.h"
-#include "base/memory/ref_counted.h"
-#include "base/threading/sequenced_worker_pool.h"
-
-namespace arc {
-
-// Trace lowmemorykiller events and report to UMA.
-//
-// If kernel lowmemorykiller is enabled, it would kill processes automatically
-// on memory pressure. ArcLowMemoryKillerMonitor listens on lowmemorykiller log
-// from kernel space and reports to UMA.
-//
-// Note: There should be only one ArcLowMemoryKillerMonitor instance during the
-// power cycle of a Chrome OS device, otherwise an lowmemorykiller event would
-// be reported twice since it reads in the entire kernel log from head.
-// On Chrome OS, it should live within a main browser thread.
-class ArcLowMemoryKillerMonitor {
- public:
-  ArcLowMemoryKillerMonitor();
-  ~ArcLowMemoryKillerMonitor();
-
-  void Start();
-  void Stop();
-
- private:
-  // Keep a reference to worker_pool_ in case |this| is deleted in
-  // shutdown process while this thread returns from a blocking read.
-  static void Run(scoped_refptr<base::SequencedWorkerPool> worker_pool);
-
-  scoped_refptr<base::SequencedWorkerPool> worker_pool_;
-
-  DISALLOW_COPY_AND_ASSIGN(ArcLowMemoryKillerMonitor);
-};
-
-}  // namespace arc
-
-#endif  // CHROME_BROWSER_CHROMEOS_ARC_ARC_LOW_MEMORY_KILLER_MONITOR_H_"