Remove allocator_extension_thunks since this layer is not required

base/allocator/BUILD.gn

 # Copyright (c) 2013 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.
 
 import("//build/config/allocator.gni")
 
 declare_args() {
   # Provide a way to force disable debugallocation in Debug builds,
   # e.g. for profiling (it's more rare to profile Debug builds,
   # but people sometimes need to do that).
@@ skipping 287 matching lines @@
       configs += [ "//build/config/compiler:optimize_max" ]
     }
 
     deps += [ "//base/third_party/dynamic_annotations" ]
 
     if (is_win) {
       ldflags = [ "/ignore:4006:4221" ]
     }
   }
 }  # !is_android
-
-source_set("extension_thunks") {
-  visibility = [ "//base/*" ]
-  sources = [
-    "allocator_extension_thunks.cc",
-    "allocator_extension_thunks.h",
-  ]
-  if (is_android && !is_debug) {
-    configs -= [ "//build/config/compiler:default_optimization" ]
-    configs += [ "//build/config/compiler:optimize_max" ]
-  }
-}