Remove allocator_extension_thunks since this layer is not required

base/allocator/allocator_extension_thunks.h

-// Copyright (c) 2012 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_EXTENSION_THUNKS_H_
-#define BASE_ALLOCATOR_ALLOCATOR_EXTENSION_THUNKS_H_
-
-#include <stddef.h> // for size_t
-
-namespace base {
-namespace allocator {
-namespace thunks {
-
-// WARNING: You probably don't want to use this file unless you are routing a
-// new allocator extension from a specific allocator implementation to base.
-// See allocator_extension.h to see the interface that base exports.
-
-typedef void (*ReleaseFreeMemoryFunction)();
-void SetReleaseFreeMemoryFunction(
-    ReleaseFreeMemoryFunction release_free_memory_function);
-ReleaseFreeMemoryFunction GetReleaseFreeMemoryFunction();
-
-typedef bool (*GetNumericPropertyFunction)(const char* name, size_t* value);
-void SetGetNumericPropertyFunction(
-    GetNumericPropertyFunction get_numeric_property_function);
-GetNumericPropertyFunction GetGetNumericPropertyFunction();
-
-}  // namespace thunks
-}  // namespace allocator
-}  // namespace base
-
-#endif  // BASE_ALLOCATOR_ALLOCATOR_EXTENSION_THUNKS_H_