Update from https://crrev.com/316786

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 {
-    auto result = shared_memory_->Lock(0, 0);
-    DCHECK_NE(result, DiscardableSharedMemory::PURGED);
-    return result == DiscardableSharedMemory::SUCCESS;
+  bool Lock() override { return false; }
+  void Unlock() override {
+    shared_memory_->Unlock(0, 0);
+    shared_memory_.reset();
   }
-  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
@@ skipping 32 matching lines @@
 // static
 DiscardableMemoryShmemAllocator*
 DiscardableMemoryShmemAllocator::GetInstance() {
   if (!g_allocator)
     g_allocator = g_default_allocator.Pointer();
 
   return g_allocator;
 }
 
 }  // namespace base