[NOT FOR REVIEW] Basic allocator shim for macOS.

build/config/allocator.gni

Index: build/config/allocator.gni
diff --git a/build/config/allocator.gni b/build/config/allocator.gni
index f723e4d48ce4f6028e7e26e852ddedafc90bbb15..19cb7d9d2129c9894da55830e034e8711fd65e72 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
@@ -43,9 +44,10 @@ assert(use_allocator == "none" || use_allocator == "tcmalloc")
 
 assert(!is_win || use_allocator == "none", "Tcmalloc doesn't work on Windows.")
 
-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,