Move memory_coordinator_client and its registry to base/memory

base/memory/memory_coordinator_client_registry.h

Index: base/memory/memory_coordinator_client_registry.h
diff --git a/base/memory/memory_coordinator_client_registry.h b/base/memory/memory_coordinator_client_registry.h
new file mode 100644
index 0000000000000000000000000000000000000000..701abb8399639dfb058467edf22f2028945874b4
--- /dev/null
+++ b/base/memory/memory_coordinator_client_registry.h
@@ -0,0 +1,41 @@
+// 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.
+
+#ifndef BASE_MEMORY_MEMORY_CLIENT_REGISTRY_H_
+#define BASE_MEMORY_MEMORY_CLIENT_REGISTRY_H_
+
+#include "base/base_export.h"
+#include "base/memory/memory_coordinator_client.h"
+#include "base/memory/singleton.h"
+#include "base/observer_list_threadsafe.h"
+
+namespace base {
+
+// MemoryCoordinatorClientRegistry is the registry for
+// MemoryCoordinatorClients. Callbacks of MemoryCoordinatorClient are called
+// via MemoryCoordinator.
+class BASE_EXPORT MemoryCoordinatorClientRegistry {
+ public:
+  static MemoryCoordinatorClientRegistry* GetInstance();
+
+  ~MemoryCoordinatorClientRegistry();
+
+  // Registers/unregisters a client. Does not take ownership of client.
+  void Register(MemoryCoordinatorClient* client);
+  void Unregister(MemoryCoordinatorClient* client);
+
+  using ClientList = ObserverListThreadSafe<MemoryCoordinatorClient>;
+  ClientList* clients() { return clients_.get(); }
+
+ private:
+  friend struct DefaultSingletonTraits<MemoryCoordinatorClientRegistry>;
+
+  MemoryCoordinatorClientRegistry();
+
+  scoped_refptr<ClientList> clients_;
+};
+
+}  // namespace base
+
+#endif  // BASE_MEMORY_MEMORY_CLIENT_REGISTRY_H_