Expose a lower bound of malloc'd memory via heap statistics

src/base/accounting-allocator.h

+// Copyright 2016 the V8 project 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 V8_BASE_ACCOUNTING_ALLOCATOR_H_
+#define V8_BASE_ACCOUNTING_ALLOCATOR_H_
+
+#include "src/base/atomicops.h"
+#include "src/base/macros.h"
+
+namespace v8 {
+namespace base {
+
+class AccountingAllocator final {
+ public:
+  AccountingAllocator() = default;
+  ~AccountingAllocator() = default;
+
+  // Returns nullptr on failed allocation.
+  void* Allocate(size_t bytes);
+  void Free(void* memory, size_t bytes);
+
+  size_t GetCurrentMemoryUsage() const;
+
+ private:
+  AtomicWord current_memory_usage_ = 0;
+
+  DISALLOW_COPY_AND_ASSIGN(AccountingAllocator);
+};
+
+}  // namespace base
+}  // namespace v8
+
+#endif  // V8_BASE_ACCOUNTING_ALLOCATOR_H_