mac: Hook up allocator shim.

build/config/allocator.gni

 # Copyright 2014 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/sanitizers/sanitizers.gni")
 
 # Temporarily disable tcmalloc on arm64 linux to get rid of compilation errors.
 if (is_android || current_cpu == "mipsel" || is_mac || is_ios || is_asan ||
     is_lsan || is_tsan || is_msan || is_win || is_syzyasan ||
     (is_linux && target_cpu == "arm64")) {
   _default_allocator = "none"
 } else {
   _default_allocator = "tcmalloc"
 }
 
 # The debug CRT on Windows has some debug features that are incompatible with
 # the shim. NaCl in particular does seem to link some binaries statically
 # against the debug CRT with "is_nacl=false".
-if ((is_linux || is_android || (is_win && !is_component_build && !is_debug)) &&
-     !is_asan && !is_lsan && !is_tsan && !is_msan) {
+if ((is_linux || is_android || is_mac ||

[Primiano Tucci (use gerrit), 2017/01/17 17:09:59]

the pattern I used in the past was to flip the switch separately. 
In the past this always broke some random bot in a random waterfall and took a
bunch of attempts to stick.
Having the flag in a separate CL avoids revert/reland thrashing on the rest of
the code 

+     (is_win && !is_component_build && !is_debug)) && !is_asan && !is_lsan &&
+    !is_tsan && !is_msan) {
   _default_use_experimental_allocator_shim = true
 } else {
   _default_use_experimental_allocator_shim = false
 }
 
 declare_args() {
   # Memory allocator to use. Set to "none" to use default allocator.
   use_allocator = _default_allocator
 
   # TODO(primiano): this should just become the default without having a flag,
   # but we need to get there first. http://crbug.com/550886 .
   # Causes all the allocations to be routed via allocator_shim.cc.
   use_experimental_allocator_shim = _default_use_experimental_allocator_shim
 }
 
 if (is_nacl) {
   # Turn off the build flag for NaCL builds to minimize confusion, as NaCL
   # doesn't support the heap shim.
   use_experimental_allocator_shim = false
 }
 
 assert(use_allocator == "none" || use_allocator == "tcmalloc")
 
 assert(!is_win || use_allocator == "none", "Tcmalloc doesn't work on Windows.")
+assert(!is_mac || use_allocator == "none", "Tcmalloc doesn't work on macOS.")
 
-assert(
-    !use_experimental_allocator_shim || is_linux || is_android || is_win,
-    "use_experimental_allocator_shim supported only on Linux, Android and Windows targets")
+assert(!use_experimental_allocator_shim || is_linux || is_android || is_win ||
+           is_mac,
+       "use_experimental_allocator_shim supported only on Linux, Android, " +
+           "Windows and macOS targets")
 
 if (is_win && use_experimental_allocator_shim) {
   assert(!is_component_build,
          "The allocator shim doesn't work for the component build on Windows.")
 }