Turn on allocator shim on macOS.

build/config/allocator.gni

Index: build/config/allocator.gni
diff --git a/build/config/allocator.gni b/build/config/allocator.gni
index 68f4e6b60d338386a7d2772e15c2754a5abdcd04..6c3b48f70a3809800c55ccb82b326ad7c5df3b88 100644
--- a/build/config/allocator.gni
+++ b/build/config/allocator.gni
@@ -16,8 +16,9 @@ if (is_android || current_cpu == "mipsel" || is_mac || is_ios || is_asan ||
 # 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 ||
+     (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