SDK: Improve the C++ SDK readme, and add some missing docstrings.

sdk_build/data/common/download_file_from_google_storage.py

 #!/usr/bin/env python
 # Copyright 2016 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.
 
-#FIXME
+"""Script for downloading a file from Google Cloud Storage."""
+
 
 import argparse
 import hashlib
 import httplib
 import os
 import sys
 
 
 def _FatalError(message):
   print >> sys.stderr, "%s: fatal error: %s" % (os.path.basename(sys.argv[0]),
                                                 message)
 
 
 def _DownloadFile(source):
-  """FIXME"""
+  """Downloads |source| from Google Cloud Storage. |source| should be a path,
+  including the bucket name (e.g., if the full URL is gs://mojo/a/b/c.xyz,
+  |source| should be "mojo/a/b/c.xyz")."""
 
   try:
     conn = httplib.HTTPSConnection("storage.googleapis.com")
     conn.request("GET", "/" + source)
     resp = conn.getresponse()
     if resp.status != httplib.OK:
       _FatalError("HTTP status: %s" % resp.reason)
     data = resp.read()
     conn.close()
     return data
   except httplib.HTTPException as e:
     _FatalError("HTTP exception: %s" % str(e))
 
 
 def main():
   parser = argparse.ArgumentParser(
       description="Downloads a file from Google Cloud Storage.")
   parser.add_argument("--sha1-hash", dest="sha1_hash",
                       help="SHA-1 hash for the downloaded file")
   parser.add_argument("--executable", action="store_true",
                       help="make the downloaded file executable")
-  parser.add_argument("source", help="source path, including bucket name")
+  parser.add_argument("source", help="source path, including the bucket name")
   parser.add_argument("destination", help="destination path")
   args = parser.parse_args()
 
   bits = _DownloadFile(args.source)
   if args.sha1_hash:
     got_sha1_hash = hashlib.sha1(bits).hexdigest().lower()
     expected_sha1_hash = args.sha1_hash.lower()
     if got_sha1_hash != expected_sha1_hash:
       _FatalError("SHA-1 hash did not match: got %s, expected %s" %
                   (got_sha1_hash, expected_sha1_hash))
 
   with open(args.destination, "wb") as f:
     f.write(bits)
 
   if args.executable:
     curr_mode = os.stat(args.destination).st_mode
     # Set the x bits (0111) where the r bits (0444) are set.
     new_mode = curr_mode | ((curr_mode & 0444) >> 2)
     os.chmod(args.destination, new_mode)
 
   return 0
 
 
 if __name__ == "__main__":
   sys.exit(main())