Update from https://crrev.com/309717

base/memory/discardable_memory_shmem_allocator.cc

 // Copyright 2014 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/discardable_memory_shmem_allocator.h"
 
 #include "base/lazy_instance.h"
 #include "base/logging.h"
 #include "base/memory/discardable_shared_memory.h"
 
 namespace base {
 namespace {
 
+class DiscardableMemoryShmemChunkImpl : public DiscardableMemoryShmemChunk {
+ public:
+  explicit DiscardableMemoryShmemChunkImpl(
+      scoped_ptr<DiscardableSharedMemory> shared_memory)
+      : shared_memory_(shared_memory.Pass()) {}
+
+  // Overridden from DiscardableMemoryShmemChunk:
+  bool Lock() override { return shared_memory_->Lock(0, 0); }
+  void Unlock() override { shared_memory_->Unlock(0, 0); }
+  void* Memory() const override { return shared_memory_->memory(); }
+  bool IsMemoryResident() const override {
+    return shared_memory_->IsMemoryResident();
+  }
+
+ private:
+  scoped_ptr<DiscardableSharedMemory> shared_memory_;
+
+  DISALLOW_COPY_AND_ASSIGN(DiscardableMemoryShmemChunkImpl);
+};
+
 // Default allocator implementation that allocates in-process
 // DiscardableSharedMemory instances.
 class DiscardableMemoryShmemAllocatorImpl
     : public DiscardableMemoryShmemAllocator {
  public:
   // Overridden from DiscardableMemoryShmemAllocator:
-  virtual scoped_ptr<DiscardableSharedMemory>
-  AllocateLockedDiscardableSharedMemory(size_t size) override {
+  scoped_ptr<DiscardableMemoryShmemChunk>
+  AllocateLockedDiscardableMemory(size_t size) override {
     scoped_ptr<DiscardableSharedMemory> memory(new DiscardableSharedMemory);
     if (!memory->CreateAndMap(size))
-      return scoped_ptr<DiscardableSharedMemory>();
+      return nullptr;
 
-    return memory.Pass();
+    return make_scoped_ptr(new DiscardableMemoryShmemChunkImpl(memory.Pass()));
   }
 };
 
 LazyInstance<DiscardableMemoryShmemAllocatorImpl>::Leaky g_default_allocator =
     LAZY_INSTANCE_INITIALIZER;
 
 DiscardableMemoryShmemAllocator* g_allocator = nullptr;
 
 }  // namespace
 
@@ skipping 12 matching lines @@
 // static
 DiscardableMemoryShmemAllocator*
 DiscardableMemoryShmemAllocator::GetInstance() {
   if (!g_allocator)
     g_allocator = g_default_allocator.Pointer();
 
   return g_allocator;
 }
 
 }  // namespace base