Adds MemoryPressureListener.

base/memory/memory_pressure_listener.cc

Index: base/memory/memory_pressure_listener.cc
diff --git a/base/memory/memory_pressure_listener.cc b/base/memory/memory_pressure_listener.cc
new file mode 100644
index 0000000000000000000000000000000000000000..b0b4161751fc92de56e4e44ed0d53d2b6c85e6e3
--- /dev/null
+++ b/base/memory/memory_pressure_listener.cc
@@ -0,0 +1,40 @@
+// 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.
+
+#include "base/memory/memory_pressure_listener.h"
+
+#include "base/lazy_instance.h"
+#include "base/observer_list_threadsafe.h"
+
+namespace {
+
+base::LazyInstance<ObserverListThreadSafe<base::MemoryPressureListener> >::Leaky
+    g_observers = LAZY_INSTANCE_INITIALIZER;
+
+}  // namespace
+
+namespace base {
+
+MemoryPressureListener::MemoryPressureListener(
+    const MemoryPressureListener::MemoryPressureCallback& callback)
+    : callback_(callback) {
+  g_observers.Get().AddObserver(this);
+}
+
+MemoryPressureListener::~MemoryPressureListener() {
+  g_observers.Get().RemoveObserver(this);
+}
+
+void MemoryPressureListener::Notify(MemoryPressureLevel memory_pressure_level) {
+  callback_.Run(memory_pressure_level);
+}
+
+// static
+void MemoryPressureListener::NotifyMemoryPressure(
+    MemoryPressureLevel memory_pressure_level) {
+  g_observers.Get().Notify(&MemoryPressureListener::Notify,
+                           memory_pressure_level);
+}
+
+}  // namespace base