Update from https://crrev.com/308331

base/chromeos/memory_pressure_observer_chromeos.h

+// Copyright 2014 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_CHROMEOS_MEMORY_PRESSURE_OBSERVER_CHROMEOS_H_
+#define BASE_CHROMEOS_MEMORY_PRESSURE_OBSERVER_CHROMEOS_H_
+
+#include "base/base_export.h"
+#include "base/macros.h"
+#include "base/memory/memory_pressure_listener.h"
+#include "base/timer/timer.h"
+
+namespace base {
+
+////////////////////////////////////////////////////////////////////////////////
+// MemoryPressureObserverChromeOS
+//
+// A class to handle the observation of our free memory. It notifies the
+// MemoryPressureListener of memory fill level changes, so that it can take
+// action to reduce memory resources accordingly.
+//
+class BASE_EXPORT MemoryPressureObserverChromeOS {
+ public:
+  // This is like the |MemoryPressureListener::MemoryPressureLevel| but it has
+  // more states to allow for a finer grained control as well as a request for
+  // the current status.
+  // TODO(skuhne): If the |MemoryPressureListener| will get extended to support
+  // all these levels, this can be removed.
+  enum MemoryPressureLevel {
+    // There is enough memory available to use.
+    MEMORY_PRESSURE_LEVEL_LOW = 0,
+
+    // Modules are advised to free buffers that are cheap to re-allocate and not
+    // immediately needed.
+    MEMORY_PRESSURE_LEVEL_MODERATE = 1,
+
+    // Modules are advised that they might get unloaded dependent on the OS.
+    // As such they should start to release more memory if possible.
+    MEMORY_PRESSURE_LEVEL_HIGH = 2,
+
+    // At this level, modules are advised to free all possible memory.  The
+    // alternative is to be killed by the system, which means all memory will
+    // have to be re-created, plus the cost of a cold start.
+    MEMORY_PRESSURE_LEVEL_CRITICAL = 3,
+  };
+
+  MemoryPressureObserverChromeOS();
+  ~MemoryPressureObserverChromeOS();
+
+  // Get the current memory pressure level.
+  MemoryPressureLevel GetCurrentPressureLevel() {
+    return current_memory_pressure_level_;
+  }
+
+ private:
+  // Starts observing the memory fill level.
+  // Calls to StartObserving should always be matched with calls to
+  // StopObserving.
+  void StartObserving();
+
+  // Stop observing the memory fill level.
+  // May be safely called if StartObserving has not been called.
+  void StopObserving();
+
+  // The function which gets periodically be called to check any changes in the
+  // memory pressure.
+  void CheckMemoryPressure();
+
+  // The current memory pressure.
+  MemoryPressureLevel current_memory_pressure_level_;
+
+  // A periodic timer to check for resource pressure changes. This will get
+  // replaced by a kernel triggered event system (see crbug.com/381196).
+  base::RepeatingTimer<MemoryPressureObserverChromeOS> timer_;
+
+  DISALLOW_COPY_AND_ASSIGN(MemoryPressureObserverChromeOS);
+};
+
+}  // namespace base
+
+#endif  // BASE_CHROMEOS_MEMORY_PRESSURE_OBSERVER_CHROMEOS_H_