Adds MemoryPressureListener.

base/memory/memory_pressure.h

+// Copyright (c) 2013 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.
+
+// MemoryPressure provides static APIs for handling memory pressure on
+// platforms that have such signals, such as Android.
+// The app will try to discard buffers that aren't deemed essential (individual
+// modules will implement their own policy).
+//
+// Note that the platform would potentially kill the app in order to free memory
+// that other apps may be requesting.
+// The trade-off here is in then terms of how long does it take to free such
+// memory, and later on potentially re-create it, versus being killed and
+// restarted from cold.

[Peter Kasting, 2013/06/07 01:00:43]

This paragraph has grammar problems and is confusing anyway when compared
against the level list; how about just "Refer to memory_pressure_level_list.h
for information about what sorts of signals can be sent under what conditions."

[bulach, 2013/06/07 11:06:58]

Done.

+
+#ifndef BASE_MEMORY_PRESSURE_H_
+#define BASE_MEMORY_PRESSURE_H_
+
+#include "base/base_export.h"
+#include "base/basictypes.h"
+#include "base/memory/ref_counted.h"
+#include "base/memory/singleton.h"
+#include "base/observer_list_threadsafe.h"
+
+namespace base {
+
+// To start listening, create a new instance, passing a callback to a
+// function that takes an MemoryPressureLevel parameter. To stop listening,

[Peter Kasting, 2013/06/07 01:00:43]

Nit: an -> a

[bulach, 2013/06/07 11:06:58]

Done.

+// simply delete the listener object. The implementation guarantees
+// that the callback will always be called on the thread that created
+// the listener.
+// If this is the same thread as the system is broadcasting the memory pressure
+// event on, then it is guaranteed you're called synchronously within that
+// broadcast and hence you should not do long-running garbage collection work.
+// But conversely, if there's something that needs to be released before
+// it returns control back to system code, this is the place to do it.

[Peter Kasting, 2013/06/07 01:00:43]

Nit: it returns control back -> control is returned

[bulach, 2013/06/07 11:06:58]

Done.

+// Please see notes on memory_pressure_level_list.h: some levels are absolutely
+// critical, and if not enough memory is returned to the system, it'll
+// potentially kill the app, and then later it'll have to be restarted from

[Peter Kasting, 2013/06/07 01:00:43]

Nit: it'll have to be restarted from cold -> Chrome will have to be cold-started

[bulach, 2013/06/07 11:06:58]

Done.

+// cold.
+//
+//
+// Example:
+//
+//    void OnMemoryPressure(MemoryPressureLevel memory_pressure_level) {
+//       ...
+//    }
+//
+//    // Start listening.
+//    MemoryPressure::Listener* my_listener =
+//        new MemoryPressure::Listener(base::Bind(&OnMemoryPressure));
+//
+//    ...
+//
+//    // Stop listening.
+//    delete my_listener

[Peter Kasting, 2013/06/07 01:00:43]

Nit: semicolon

[bulach, 2013/06/07 11:06:58]

Done.

+//
+class BASE_EXPORT MemoryPressure {

[Peter Kasting, 2013/06/07 01:00:43]

Nit: I'd call this class "MemoryPressureHandler", as "memory pressure" is a
condition, not an object.

[bulach, 2013/06/07 11:06:58]

please see joth's nit, it was a handler before:
https://codereview.chromium.org/15995014/diff/5001/base/memory/memory_pressur...

I'm happy to go either way but I can't make you both happy :)
please let me know who feels stronger about this.

+ public:
+  enum MemoryPressureLevel {
+#define DEFINE_MEMORY_PRESSURE_LEVEL(name, value) name = value,
+#include "base/memory/memory_pressure_level_list.h"
+  };
+
+  typedef base::Callback<void(MemoryPressureLevel)> MemoryPressureCallback;
+
+  class Listener {
+   public:
+    explicit Listener(const MemoryPressureCallback& memory_pressure_callback);
+    ~Listener();
+
+   private:
+    friend class MemoryPressure;

[Peter Kasting, 2013/06/07 01:00:43]

I see no reason to make Notify() private and make the outer class a friend. 
Just make this public.

[bulach, 2013/06/07 11:06:58]

Done.

+
+    void Notify(MemoryPressureLevel memory_pressure_level);
+
+    MemoryPressureCallback callback_;
+
+    DISALLOW_COPY_AND_ASSIGN(Listener);
+  };
+
+  static MemoryPressure* GetInstance();
+
+  // Internal use only: called when the platform receives a memory pressure
+  // signal.
+  void NotifyMemoryPressure(MemoryPressureLevel memory_pressure_level);
+
+ private:
+  friend struct DefaultSingletonTraits<MemoryPressure>;
+
+  MemoryPressure();
+  ~MemoryPressure();
+
+  void RegisterListener(Listener* listener);
+  void UnregisterListener(Listener* listener);
+
+  scoped_refptr<ObserverListThreadSafe<Listener> > observers_;
+
+  DISALLOW_COPY_AND_ASSIGN(MemoryPressure);
+};
+
+}  // namespace base
+
+#endif  // BASE_MEMORY_PRESSURE_H_