Adds MemoryPressureListener.

base/memory/memory_pressure.cc

Index: base/memory/memory_pressure.cc
diff --git a/base/memory/memory_pressure.cc b/base/memory/memory_pressure.cc
new file mode 100644
index 0000000000000000000000000000000000000000..ae5862ac9f44756f6140061937229de0974b04f1
--- /dev/null
+++ b/base/memory/memory_pressure.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.h"
+
+#include "base/memory/singleton.h"
+
+namespace base {
+
+MemoryPressure::Listener::Listener(
+    const MemoryPressure::MemoryPressureCallback& callback)
+    : callback_(callback) {
+  MemoryPressure::GetInstance()->RegisterListener(this);
+}
+
+MemoryPressure::Listener::~Listener() {
+  MemoryPressure::GetInstance()->UnregisterListener(this);
+}
+
+void MemoryPressure::Listener::Notify(
+    MemoryPressureLevel memory_pressure_level) {
+  callback_.Run(memory_pressure_level);
+}
+
+// static
+MemoryPressure* MemoryPressure::GetInstance() {
+  return Singleton<MemoryPressure,
+                   LeakySingletonTraits<MemoryPressure> >::get();
+}
+
+MemoryPressure::MemoryPressure()
+    : observers_(new ObserverListThreadSafe<Listener>()) {
+}
+
+MemoryPressure::~MemoryPressure() {}
+
+void MemoryPressure::RegisterListener(Listener* listener) {
+  observers_->AddObserver(listener);
+}
+
+void MemoryPressure::UnregisterListener(Listener* listener) {
+  observers_->RemoveObserver(listener);
+}
+
+void MemoryPressure::NotifyMemoryPressure(
+    MemoryPressureLevel memory_pressure_level) {
+  observers_->Notify(
+      &MemoryPressure::Listener::Notify, memory_pressure_level);
+}
+
+}  // namespace base