Revert "base: Implement browser process support for discardable memory."

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() {
-  DCHECK(g_allocator);
+  if (!g_allocator)
+    g_allocator = g_default_allocator.Pointer();
+
   return g_allocator;
 }
 
 }  // namespace base