Pass ID of the calling extension to the native messaging host.

chrome/test/data/native_messaging/native_hosts/echo.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.
 
 # A simple native client in python.
 # All this client does is echo the text it receives back at the extension.
 
 import sys
 import struct
 
 def Main():
   message_number = 0
 
+  if len(sys.argv) < 2:
+    sys.stderr.write("URL of the calling application is not specified.\n")
+    return;
+  caller_url = sys.argv[1]
+
   while 1:
     # Read the message type (first 4 bytes).
     text_length_bytes = sys.stdin.read(4)
 
     if len(text_length_bytes) == 0:
       break
 
     # Read the message length (4 bytes).
     text_length = struct.unpack('i', text_length_bytes)[0]
 
     # Read the text (JSON object) of the message.
     text = sys.stdin.read(text_length).decode('utf-8')
 
     message_number += 1
 
-    response = '{{"id": {0}, "echo": {1}}}'.format(message_number,
-                                                   text).encode('utf-8')
+    response = '{{"id": {0}, "echo": {1}, "caller_url": "{2}"}}'.format(
+        message_number, text, caller_url).encode('utf-8')
 
     try:
       sys.stdout.write(struct.pack("I", len(response)))
       sys.stdout.write(response)
       sys.stdout.flush()
     except IOError:
       break
 
 if __name__ == '__main__':
   Main()