allocator: add use_experimental_allocator_shim build flag

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")
 import("//build/config/compiler/compiler.gni")
 
 if (is_win) {
   import("//build/config/win/visual_studio_version.gni")
 }
@@ skipping 29 matching lines @@
 # TODO(brettw) this is only used in one place and is kind of mess, because it
 # assumes that the library using it will eventually be linked with
 # //base/allocator in the default way. Clean this up and delete this.
 config("allocator_shim_define") {
   if (is_win && !is_component_build && visual_studio_version != "2015") {
     defines = [ "ALLOCATOR_SHIM" ]
   }
 }
 
 config("tcmalloc_flags") {
+  defines = []
   if (enable_debugallocation) {
-    defines = [
+    defines += [
       # Use debugallocation for Debug builds to catch problems early
       # and cleanly, http://crbug.com/30715 .
       "TCMALLOC_FOR_DEBUGALLOCATION",
     ]
   }
+  if (use_experimental_allocator_shim) {
+    defines += [ "TCMALLOC_DONT_REPLACE_SYSTEM_ALLOC" ]
+  }
   if (is_clang) {
     cflags = [
       # tcmalloc initializes some fields in the wrong order.
       "-Wno-reorder",
 
       # tcmalloc contains some unused local template specializations.
       "-Wno-unused-function",
 
       # tcmalloc uses COMPILE_ASSERT without static_assert but with
       # typedefs.
@@ skipping 216 matching lines @@
     # Make sure the allocation library is optimized as much as possible when
     # we"re in release mode.
     if (!is_debug) {
       configs -= [ "//build/config/compiler:default_optimization" ]
       configs += [ "//build/config/compiler:optimize_max" ]
     }
 
     deps += [ "//base/third_party/dynamic_annotations" ]
   }
 }  # use_allocator == "tcmalloc"