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

src/base/accounting-allocator.cc

+// 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.
+
+#include "src/base/accounting-allocator.h"
+
+#include <cstdlib>
+
+#if V8_LIBC_BIONIC
+#include <malloc.h>  // NOLINT
+#endif
+
+namespace v8 {
+namespace base {
+
+void* AccountingAllocator::Allocate(size_t bytes) {
+  void* memory = malloc(bytes);
+  if (memory) NoBarrier_AtomicIncrement(&current_memory_usage_, bytes);
+  return memory;
+}
+
+void AccountingAllocator::Free(void* memory, size_t bytes) {
+  free(memory);
+  NoBarrier_AtomicIncrement(&current_memory_usage_,
+                            -static_cast<AtomicWord>(bytes));
+}
+
+size_t AccountingAllocator::GetCurrentMemoryUsage() const {
+  return NoBarrier_Load(&current_memory_usage_);
+}
+
+}  // namespace base
+}  // namespace v8