Enable cross compile to android from mac.

build/config/android/config.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.
 
 # This file contains common system config stuff for the Android build.
 
 if (is_android) {
   has_chrome_android_internal =
       exec_script("//build/dir_exists.py",
                   [ rebase_path("//clank", root_build_dir) ],
@@ skipping 42 matching lines @@
   } else if (host_cpu == "x86") {
     android_host_arch = "x86"
   } else {
     assert(false, "Need Android toolchain support for your build CPU arch.")
   }
 
   # Defines the name the Android build gives to the current host CPU
   # architecture, which is different than the names GN uses.
   if (host_os == "linux") {
     android_host_os = "linux"
+  } else if (host_os == "mac") {
+    android_host_os = "darwin"
   } else {
     assert(false, "Need Android toolchain support for your build OS.")
   }
 
   # Directories and files ------------------------------------------------------
   #
   # We define may of the dirs strings here for each output architecture (rather
   # than just the current one) since these are needed by the Android toolchain
   # file to define toolchains for all possible targets in one pass.
 
@@ skipping 108 matching lines @@
   } else if (current_cpu == "x64") {
     android_app_abi = "x86_64"
   } else if (current_cpu == "arm64") {
     android_app_abi = "arm64-v8a"
   } else if (current_cpu == "mips64el") {
     android_app_abi = "mips64"
   } else {
     assert(false, "Unknown Android ABI: " + current_cpu)
   }
 }