Update from https://crrev.com/320931

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 false; }
-  void Unlock() override {
-    shared_memory_->Unlock(0, 0);
-    shared_memory_.reset();
-  }
-  void* Memory() const override { return shared_memory_->memory(); }
-
- 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:
-  scoped_ptr<DiscardableMemoryShmemChunk>
-  AllocateLockedDiscardableMemory(size_t size) override {
-    scoped_ptr<DiscardableSharedMemory> memory(new DiscardableSharedMemory);
-    if (!memory->CreateAndMap(size))
-      return nullptr;
-
-    return make_scoped_ptr(new DiscardableMemoryShmemChunkImpl(memory.Pass()));
-  }
-};
-
-LazyInstance<DiscardableMemoryShmemAllocatorImpl>::Leaky g_default_allocator =
-    LAZY_INSTANCE_INITIALIZER;
-
 DiscardableMemoryShmemAllocator* g_allocator = nullptr;
 
 }  // namespace
 
 // static
 void DiscardableMemoryShmemAllocator::SetInstance(
     DiscardableMemoryShmemAllocator* allocator) {
   DCHECK(allocator);
 
   // Make sure this function is only called once before the first call
   // to GetInstance().
   DCHECK(!g_allocator);
 
   g_allocator = allocator;
 }
 
 // static
 DiscardableMemoryShmemAllocator*
 DiscardableMemoryShmemAllocator::GetInstance() {
-  if (!g_allocator)
-    g_allocator = g_default_allocator.Pointer();
-
+  DCHECK(g_allocator);
   return g_allocator;
 }
 
 }  // namespace base