Introduce base::MemoryPressureMonitor

base/memory/memory_pressure_monitor.h

Index: base/memory/memory_pressure_monitor.h
diff --git a/base/memory/memory_pressure_monitor.h b/base/memory/memory_pressure_monitor.h
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+++ b/base/memory/memory_pressure_monitor.h
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+// 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.
+
+#ifndef BASE_MEMORY_MEMORY_PRESSURE_MONITOR_H_
+#define BASE_MEMORY_MEMORY_PRESSURE_MONITOR_H_
+
+#include "base/base_export.h"
+#include "base/memory/memory_pressure_listener.h"
+
+namespace base {
+
+// Declares the interface for a MemoryPressureMonitor. There are multiple
+// OS specific implementations of this class. An instance of the memory
+// pressure observer is created at the process level, tracks memory usage, and
+// pushes memory state change notifications to the static function
+// base::MemoryPressureListener::NotifyMemoryPressure. This is turn notifies
+// all MemoryPressureListener instances via a callback.
+class BASE_EXPORT MemoryPressureMonitor {
+ public:
+  using MemoryPressureLevel = base::MemoryPressureListener::MemoryPressureLevel;
+
+  // Return the singleton MemoryPressureMonitor.
+  static MemoryPressureMonitor* Get();
+
+  // Returns the currently observed memory pressure.
+  virtual MemoryPressureLevel GetCurrentPressureLevel() const = 0;
+
+ protected:
+  MemoryPressureMonitor();
+  virtual ~MemoryPressureMonitor();
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(MemoryPressureMonitor);
+};
+
+}  // namespace base
+
+#endif  // BASE_MEMORY_MEMORY_PRESSURE_MONITOR_H_