Re-land: base: Introduce an explicit call for reducing emulated discardable memory usage.

base/memory/discardable_memory_emulated.h

 // Copyright 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_EMULATED_H_
 #define BASE_MEMORY_DISCARDABLE_MEMORY_EMULATED_H_
 
 #include "base/memory/discardable_memory.h"
 
 #include "base/memory/discardable_memory_manager.h"
 
 namespace base {
 namespace internal {
 
 class DiscardableMemoryEmulated
     : public DiscardableMemory,
       public internal::DiscardableMemoryManagerAllocation {
  public:
   explicit DiscardableMemoryEmulated(size_t bytes);
   virtual ~DiscardableMemoryEmulated();
 
-  static void RegisterMemoryPressureListeners();
-  static void UnregisterMemoryPressureListeners();
   static bool ReduceMemoryUsage();
 
+  // TODO(reveman): Remove this as it is breaking the discardable memory design
+  // principle that implementations should not rely on information this is
+  // unavailable in kernel space. crbug.com/400423
+  BASE_EXPORT static void ReduceMemoryUsageUntilWithinLimit(size_t bytes);
+
   static void PurgeForTesting();
 
   bool Initialize();
 
   // Overridden from DiscardableMemory:
   virtual DiscardableMemoryLockStatus Lock() OVERRIDE;
   virtual void Unlock() OVERRIDE;
   virtual void* Memory() const OVERRIDE;
 
   // Overridden from internal::DiscardableMemoryManagerAllocation:
   virtual bool AllocateAndAcquireLock() OVERRIDE;
   virtual void ReleaseLock() OVERRIDE {}
   virtual void Purge() OVERRIDE;
 
  private:
   const size_t bytes_;
   scoped_ptr<uint8[]> memory_;
   bool is_locked_;
 
   DISALLOW_COPY_AND_ASSIGN(DiscardableMemoryEmulated);
 };
 
 }  // namespace internal
 }  // namespace base
 
 #endif  // BASE_MEMORY_DISCARDABLE_MEMORY_EMULATED_H_