Fix finding chrome binary for ChromeDriver.

chrome/test/chromedriver/chromedriver.py

 # 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.
 
 import ctypes
 import json
 
 class ChromeDriverException(Exception):
   pass
 class UnknownCommand(ChromeDriverException):
@@ skipping 12 matching lines @@
     33: SessionNotCreatedException,
     100: NoSuchSession
   }
   status = response['status']
   msg = response['value']['message']
   return exception_class_map.get(status, ChromeDriverException)(msg)
 
 class ChromeDriver(object):
   """Starts and controls a single Chrome instance on this machine."""
 
-  def __init__(self, lib_path):
+  def __init__(self, lib_path, chrome_binary=None):
     self._lib = ctypes.CDLL(lib_path)
-    self._session_id = self._ExecuteCommand('newSession')
+    if chrome_binary is None:
+      params = {}
+    else:
+      params = {
+        'desiredCapabilities': {
+          'chrome': {
+            'binary': chrome_binary
+          }
+        }
+      }
+    self._session_id = self._ExecuteCommand('newSession', params)
 
   def _ExecuteCommand(self, name, params={}, session_id=''):
     cmd = {
       'name': name,
       'parameters': params,
       'sessionId': session_id
     }
     cmd_json = json.dumps(cmd)
     response_data = ctypes.c_char_p()
     response_size = ctypes.c_uint()
     self._lib.ExecuteCommand(
         ctypes.c_char_p(cmd_json),
         ctypes.c_uint(len(cmd_json)),
         ctypes.byref(response_data),
         ctypes.byref(response_size))
     response_json = ctypes.string_at(response_data, response_size.value)
     self._lib.Free(response_data)
     response = json.loads(response_json)
     if response['status'] != 0:
       raise _ExceptionForResponse(response)
     return response['value']
 
   def _ExecuteSessionCommand(self, name, params={}):
     return self._ExecuteCommand(name, params, self._session_id)
 
   def Quit(self):
     """Quits the browser and ends the session."""
     self._ExecuteSessionCommand('quit')