Adds MemoryPressureListener.

base/memory/memory_pressure_notifier.cc

Index: base/memory/memory_pressure_notifier.cc
diff --git a/base/memory/memory_pressure_notifier.cc b/base/memory/memory_pressure_notifier.cc
new file mode 100644
index 0000000000000000000000000000000000000000..96c6cb7aeec0a041909cc9c886c6ba5dbdc5596f
--- /dev/null
+++ b/base/memory/memory_pressure_notifier.cc
@@ -0,0 +1,52 @@
+// 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_notifier.h"
+
+#include "base/memory/singleton.h"
+
+namespace base {
+
+MemoryPressureNotifier::Listener::Listener(
+    const MemoryPressureNotifier::MemoryPressureCallback& callback)
+    : callback_(callback) {
+  MemoryPressureNotifier::GetInstance()->RegisterListener(this);
+}
+
+MemoryPressureNotifier::Listener::~Listener() {
+  MemoryPressureNotifier::GetInstance()->UnregisterListener(this);
+}
+
+void MemoryPressureNotifier::Listener::Notify(
+    MemoryPressureType memory_pressure_type) {
+  callback_.Run(memory_pressure_type);
+}
+
+// static
+MemoryPressureNotifier* MemoryPressureNotifier::GetInstance() {
+  return Singleton<MemoryPressureNotifier,
+                   LeakySingletonTraits<MemoryPressureNotifier> >::get();
+}
+
+MemoryPressureNotifier::MemoryPressureNotifier()
+    : observers_(new ObserverListThreadSafe<Listener>()) {
+}
+
+MemoryPressureNotifier::~MemoryPressureNotifier() {}
+
+void MemoryPressureNotifier::RegisterListener(Listener* listener) {
+  observers_->AddObserver(listener);
+}
+
+void MemoryPressureNotifier::UnregisterListener(Listener* listener) {
+  observers_->RemoveObserver(listener);
+}
+
+void MemoryPressureNotifier::HandleMemoryPressure(
+    MemoryPressureType memory_pressure_type) {
+  observers_->Notify(
+      &MemoryPressureNotifier::Listener::Notify, memory_pressure_type);
+}
+
+}  // namespace base