Move memory_coordinator_client and its registry to base/memory

base/memory/memory_coordinator_client_registry.cc

+// Copyright 2016 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_coordinator_client_registry.h"
+
+namespace base {
+
+// static
+MemoryCoordinatorClientRegistry*
+MemoryCoordinatorClientRegistry::GetInstance() {
+  return Singleton<
+      MemoryCoordinatorClientRegistry,
+      LeakySingletonTraits<MemoryCoordinatorClientRegistry>>::get();
+}
+
+MemoryCoordinatorClientRegistry::MemoryCoordinatorClientRegistry()
+    : clients_(new ClientList) {}
+
+MemoryCoordinatorClientRegistry::~MemoryCoordinatorClientRegistry() {}
+
+void MemoryCoordinatorClientRegistry::Register(
+    MemoryCoordinatorClient* client) {
+  clients_->AddObserver(client);
+}
+
+void MemoryCoordinatorClientRegistry::Unregister(
+    MemoryCoordinatorClient* client) {
+  clients_->RemoveObserver(client);
+}
+
+}  // namespace base