Update from chromium https://crrev.com/301315

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 {
+
+// 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<DiscardableSharedMemory> memory(new DiscardableSharedMemory);
+    if (!memory->CreateAndMap(size))
+      return scoped_ptr<DiscardableSharedMemory>();
+
+    return 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();
+
+  return g_allocator;
+}
+
+}  // namespace base