Re-land: base: Fix registering of memory pressure listeners used when emulating discardable memory.

base/memory/discardable_memory.h

 // 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.
 
 #ifndef BASE_MEMORY_DISCARDABLE_MEMORY_H_
 #define BASE_MEMORY_DISCARDABLE_MEMORY_H_
 
 #include <string>
 #include <vector>
 
@@ skipping 45 matching lines @@
 //   - Linux: http://lwn.net/Articles/452035/
 //   - Mac: http://trac.webkit.org/browser/trunk/Source/WebCore/platform/mac/PurgeableBufferMac.cpp
 //          the comment starting with "vm_object_purgable_control" at
 //            http://www.opensource.apple.com/source/xnu/xnu-792.13.8/osfmk/vm/vm_object.c
 //
 // Thread-safety: DiscardableMemory instances are not thread-safe.
 class BASE_EXPORT DiscardableMemory {
  public:
   virtual ~DiscardableMemory() {}
 
+  // Call this on a thread with a MessageLoop current to allow discardable
+  // memory implementations to respond to memory pressure signals.
+  static void RegisterMemoryPressureListeners();
+
+  // Call this to prevent discardable memory implementations from responding
+  // to memory pressure signals.
+  static void UnregisterMemoryPressureListeners();
+
   // Gets the discardable memory type with a given name.
   static DiscardableMemoryType GetNamedType(const std::string& name);
 
   // Gets the name of a discardable memory type.
   static const char* GetTypeName(DiscardableMemoryType type);
 
   // Gets system supported discardable memory types. Default preferred type
   // at the front of vector.
   static void GetSupportedTypes(std::vector<DiscardableMemoryType>* types);
 
@@ skipping 35 matching lines @@
   static bool PurgeForTestingSupported();
 
   // Purge all discardable memory in the system. This call has global effects
   // across all running processes, so it should only be used for testing!
   static void PurgeForTesting();
 };
 
 }  // namespace base
 
 #endif  // BASE_MEMORY_DISCARDABLE_MEMORY_H_