Rename Native messaging hosts directory to native_hosts.

chrome/test/data/native_messaging/native_hosts/empty_app.py

 #!/usr/bin/env python
 # Copyright (c) 2012 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 native client will read full messages, but do nothing with them and
 # send no responses.
 
 import sys
 import struct
 
 while 1:
   # Read the message type (first 4 bytes).
   typeBytes = sys.stdin.read(4)
 
   if len(typeBytes) == 0:
     break
 
   # Read the message length (4 bytes).
   textLength = struct.unpack('i', sys.stdin.read(4))[0]
 
   # Read the text (JSON object) of the message.
   text = sys.stdin.read(textLength)