Add a DiscardableMemoryShmemeAllocator.

mojo/services/html_viewer/discardable_memory_allocator.h

Index: mojo/services/html_viewer/discardable_memory_allocator.h
diff --git a/mojo/services/html_viewer/discardable_memory_allocator.h b/mojo/services/html_viewer/discardable_memory_allocator.h
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+++ b/mojo/services/html_viewer/discardable_memory_allocator.h
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+// 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_allocator.h"
+
+namespace html_viewer {
+
+// A discarable memory allocator which will unallocate chunks on new
+// allocations.
+class DiscardableMemoryAllocator : public base::DiscardableMemoryAllocator {
+ public:
+  class OwnedMemoryChunk;
+
+  explicit DiscardableMemoryAllocator(size_t desired_max_memory);
+  ~DiscardableMemoryAllocator() override;
+
+  // Overridden from DiscardableMemoryAllocator:
+  scoped_ptr<base::DiscardableMemory> AllocateLockedDiscardableMemory(
+      size_t size) override;
+
+ private:
+  friend class OwnedMemoryChunk;
+
+  // Called by OwnedMemoryChunks when they are unlocked. This puts them at the
+  // end of the live_unlocked_chunks_ list and passes an iterator to their
+  // position in the unlocked chunk list.
+  std::list<OwnedMemoryChunk*>::iterator NotifyUnlocked(
+      OwnedMemoryChunk* chunk);
+
+  // Called by OwnedMemoryChunks when they are locked. This removes the passed
+  // in unlocked chunk list.
+  void NotifyLocked(std::list<OwnedMemoryChunk*>::iterator it);
+
+  // The amount of memory we can allocate before we try to free unlocked
+  // chunks. We can go over this amount if all callers keep their discardable
+  // chunks locked.
+  const size_t desired_max_memory_;
+
+  // A count of the sum of memory. Used to trigger discarding the oldest
+  // memory.
+  size_t total_live_memory_;
+
+  // The number of locked chunks.
+  int locked_chunks_;
+
+  // A linked list of unlocked allocated chunks so that the tail is most
+  // recently accessed chunks.
+  std::list<OwnedMemoryChunk*> live_unlocked_chunks_;
+
+  DISALLOW_COPY_AND_ASSIGN(DiscardableMemoryAllocator);
+};
+
+}  // namespace html_viewer
+
+#endif  // MOJO_SERVICES_HTML_VIEWER_DISCARDABLE_MEMORY_ALLOCATOR_H_