Consolidate code monitoring low memory kills and OOM kills to MemoryKillsMonitor on ChromeOS.

chrome/browser/memory/memory_kills_histogram.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_MEMORY_MEMORY_KILLS_HISTOGRAM_H_
+#define CHROME_BROWSER_MEMORY_MEMORY_KILLS_HISTOGRAM_H_
+
+#include "base/metrics/histogram.h"
+#include "base/time/time.h"
+
+namespace memory {
+
+constexpr base::TimeDelta kMaxMemoryKillTimeDelta =
+        base::TimeDelta::FromSeconds(30);
+
+}  // namespace memory
+
+// Use this macro to report elapsed time since last Memory kill event.
+// Must be a macro as the underlying HISTOGRAM macro creates static variables.
+#define UMA_HISTOGRAM_MEMORY_KILL_TIME_INTERVAL(name, sample)  \
+  UMA_HISTOGRAM_CUSTOM_TIMES(                               \
+      name, sample, base::TimeDelta::FromMilliseconds(1),   \
+      ::memory::kMaxMemoryKillTimeDelta, 50)
+
+#endif  // CHROME_BROWSER_MEMORY_MEMORY_KILLS_HISTOGRAM_H_