Hook up allocator shim on mac.

base/allocator/allocator_shim_default_dispatch_to_mac_zoned_malloc.h

+// Copyright 2017 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 BASE_ALLOCATOR_ALLOCATOR_SHIM_DEFAULT_DISPATCH_TO_ZONED_MALLOC_H_
+#define BASE_ALLOCATOR_ALLOCATOR_SHIM_DEFAULT_DISPATCH_TO_ZONED_MALLOC_H_
+
+#include "base/allocator/allocator_interception_mac.h"
+
+namespace base {
+namespace allocator {
+
+// This initializes AllocatorDispatch::default_dispatch by saving pointers to
+// the functions in the current default malloc zone. This must be called before
+// the default malloc zone is changed to have its intended effect.
+void InitializeDefaultDispatchToMacAllocator();
+
+}  // namespace allocator
+}  // namespace base
+
+#endif  // BASE_ALLOCATOR_ALLOCATOR_SHIM_DEFAULT_DISPATCH_TO_ZONED_MALLOC_H_