Adds MemoryPressureListener.

base/memory/memory_pressure_handler.cc

Index: base/memory/memory_pressure_handler.cc
diff --git a/base/memory/memory_pressure_handler.cc b/base/memory/memory_pressure_handler.cc
new file mode 100644
index 0000000000000000000000000000000000000000..8bd90dcb9679a92bb984eeb9d72290c709589264
--- /dev/null
+++ b/base/memory/memory_pressure_handler.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_handler.h"
+
+#include "base/memory/singleton.h"
+
+namespace base {
+
+MemoryPressureHandler::Listener::Listener(
+    const MemoryPressureHandler::MemoryPressureCallback& callback)
+    : callback_(callback) {
+  MemoryPressureHandler::GetInstance()->RegisterListener(this);
+}
+
+MemoryPressureHandler::Listener::~Listener() {
+  MemoryPressureHandler::GetInstance()->UnregisterListener(this);
+}
+
+void MemoryPressureHandler::Listener::Notify(
+    MemoryPressureType memory_pressure_type) {
+  callback_.Run(memory_pressure_type);
+}
+
+// static
+MemoryPressureHandler* MemoryPressureHandler::GetInstance() {
+  return Singleton<MemoryPressureHandler,
+                   LeakySingletonTraits<MemoryPressureHandler> >::get();
+}
+
+MemoryPressureHandler::MemoryPressureHandler()
+    : observers_(new ObserverListThreadSafe<Listener>()) {
+}
+
+MemoryPressureHandler::~MemoryPressureHandler() {}
+
+void MemoryPressureHandler::RegisterListener(Listener* listener) {
+  observers_->AddObserver(listener);
+}
+
+void MemoryPressureHandler::UnregisterListener(Listener* listener) {
+  observers_->RemoveObserver(listener);
+}
+
+void MemoryPressureHandler::HandleMemoryPressure(
+    MemoryPressureType memory_pressure_type) {
+  observers_->Notify(
+      &MemoryPressureHandler::Listener::Notify, memory_pressure_type);
+}
+
+}  // namespace base