Add Mac NDK asset, and fetch NDK on Android compile bots.

infra/bots/assets/android_ndk_darwin/create.py

 #!/usr/bin/env python
 #
 # Copyright 2016 Google Inc.
 #
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 
 """Create the asset."""
 
 
 import argparse
 import glob
 import os.path
 import shutil
 import subprocess
 
 NDK_VER = "android-ndk-r12b"
 NDK_URL = \
-    "https://dl.google.com/android/repository/%s-linux-x86_64.zip" % NDK_VER
+    "https://dl.google.com/android/repository/%s-darwin-x86_64.zip" % NDK_VER
 
 def create_asset(target_dir):
   """Create the asset."""
   subprocess.check_call(["curl", NDK_URL, "-o", "ndk.zip"])
   subprocess.check_call(["unzip", "ndk.zip", "-d", target_dir])
   for f in glob.glob(os.path.join(target_dir, NDK_VER, "*")):
     shutil.move(f, target_dir)
   subprocess.check_call(["rm", "ndk.zip"])
 
 
 def main():
   parser = argparse.ArgumentParser()
   parser.add_argument('--target_dir', '-t', required=True)
   args = parser.parse_args()
   create_asset(args.target_dir)
 
 
 if __name__ == '__main__':
   main()