Fix adb path being used in `mojo_run`.

mojo/devtools/common/devtoolslib/shell_arguments.py

 # Copyright 2015 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.
 
 """Functions that configure the shell before it is run manipulating its argument
 list.
 """
 
 import os.path
 import sys
@@ skipping 168 matching lines @@
     arguments.append(key + value)
 
   return arguments
 
 
 def AddShellArguments(parser):
   """Adds argparse arguments allowing to configure shell abstraction using
   ConfigureShell() below.
   """
   # Arguments indicating paths to binaries and tools.
-  parser.add_argument('--adb-path', default='adb',
-                      help='Path of the adb binary.')
+  parser.add_argument('--adb-path', help='Path of the adb binary.')
   parser.add_argument('--shell-path', help='Path of the Mojo shell binary.')
   parser.add_argument('--origin-path', help='Path of a directory to be set as '
                       'the origin for mojo: urls')
 
   # Arguments configuring the shell run.
   parser.add_argument('--android', help='Run on Android',
                       action='store_true')
   parser.add_argument('--origin', help='Origin for mojo: URLs.')
   parser.add_argument('--map-url', action='append',
                       help='Define a mapping for a url in the format '
@@ skipping 64 matching lines @@
     if config_args.use_osmesa:
       shell_args.append('--args-for=mojo:native_viewport_service --use-osmesa')
 
   if config_args.origin:
     shell_args.append('--origin=' + config_args.origin)
   elif config_args.origin_path:
     shell_args.extend(ConfigureLocalOrigin(shell, config_args.origin_path,
                                            fixed_port=True))
 
   return shell, shell_args