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
new file mode 100644
index 0000000000000000000000000000000000000000..d9c3f7d0f39028a3f11608cd4e0206a457543204
--- /dev/null
+++ b/mojo/services/html_viewer/discardable_memory_allocator.h
@@ -0,0 +1,50 @@
+// 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 max_memory);
+  ~DiscardableMemoryAllocator() override;
+
+  // Overridden from DiscardableMemoryAllocator:
+  scoped_ptr<base::DiscardableMemory> 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_