Add a DiscardableMemoryShmemeAllocator.

mojo/services/html_viewer/discardable_memory_allocator.h

+// Copyright 2015 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.
+
+#ifndef MOJO_SERVICES_HTML_VIEWER_DISCARDABLE_MEMORY_ALLOCATOR_H_
+#define MOJO_SERVICES_HTML_VIEWER_DISCARDABLE_MEMORY_ALLOCATOR_H_
+
+#include <list>
+
+#include "base/memory/discardable_memory_shmem_allocator.h"
+
+namespace html_viewer {
+
+// A discarable memory allocator which will unallocate chunks on new
+// allocations.
+class DiscardableMemoryAllocator
+    : public base::DiscardableMemoryShmemAllocator {
+ public:
+  class OwnedMemoryChunk;

[jamesr, 2015/03/17 23:26:19]

does this forward declaration need to be public?

[Elliot Glaysher, 2015/03/18 20:36:22]

It does if I want to hide the implementation of DiscardableMemoryChunkImpl from
the linker. I'm fine moving that to an inner class of DiscardableMemoryAllocator
instead if you want, but I think this is slightly better.

+
+  DiscardableMemoryAllocator(size_t max_memory);

[jamesr, 2015/03/17 23:26:19]

explicit

+  ~DiscardableMemoryAllocator() override;
+
+  // Overridden from DiscardableMemoryShmemAllocator:
+  scoped_ptr<base::DiscardableMemoryShmemChunk> AllocateLockedDiscardableMemory(
+      size_t size) override;
+
+ private:
+  friend class OwnedMemoryChunk;
+
+  void MoveToBack(std::list<OwnedMemoryChunk*>::iterator it);
+
+  // The amount of memory we can allocate before we try to free unlocked chunks.
+  const size_t max_memory_;
+
+  // A count of the sum of memory. Used to trigger discarding the oldest
+  // memory.
+  size_t total_live_memory_;
+
+  // A linked list of allocated chunks so that the tail is the newest chunks,
+  // and the head is the oldest chunks. On Lock()ing a chunk, it is sent to the
+  // back of this list.
+  std::list<OwnedMemoryChunk*> live_chunks_;
+
+  DISALLOW_COPY_AND_ASSIGN(DiscardableMemoryAllocator);
+};
+
+}  // namespace html_viewer
+
+#endif  // MOJO_SERVICES_HTML_VIEWER_DISCARDABLE_MEMORY_ALLOCATOR_H_