memory coordinator: Add MemoryCoordinatorClient::OnPurgeMemory()

base/memory/memory_coordinator_client_registry.h

 // 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"
@@ skipping 21 matching lines @@
 
   ~MemoryCoordinatorClientRegistry();
 
   // Registers/unregisters a client. Does not take ownership of client.
   void Register(MemoryCoordinatorClient* client);
   void Unregister(MemoryCoordinatorClient* client);
 
   // Notify clients of a memory state change.
   void Notify(MemoryState state);
 
+  // Requests purging memory.
+  void PurgeMemory();
+
  private:
   friend struct DefaultSingletonTraits<MemoryCoordinatorClientRegistry>;
 
   MemoryCoordinatorClientRegistry();
 
   using ClientList = ObserverListThreadSafe<MemoryCoordinatorClient>;
   scoped_refptr<ClientList> clients_;
 };
 
 }  // namespace base
 
 #endif  // BASE_MEMORY_MEMORY_CLIENT_REGISTRY_H_