Have 'adb' as the default value for when adb_path is not specified

mojo/devtools/common/devtoolslib/shell_config.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.
 
 """Configuration for the shell abstraction.
 
 This module declares ShellConfig and knows how to compute it from command-line
 arguments, applying any default paths inferred from the checkout, configuration
 file, etc.
 """
@@ skipping 134 matching lines @@
 
   shell_config.android = script_args.android
   shell_config.shell_path = (script_args.shell_path or
                              inferred_paths['shell_path'])
   shell_config.origin = script_args.origin
   shell_config.map_url_list = script_args.map_url
   shell_config.map_origin_list = script_args.map_origin
   shell_config.verbose = script_args.verbose
 
   # Android-only.
-  shell_config.adb_path = (script_args.adb_path or inferred_paths['adb_path'])
+  shell_config.adb_path = (script_args.adb_path or inferred_paths['adb_path'] or
+                           'adb')
   shell_config.target_device = script_args.target_device
   shell_config.logcat_tags = script_args.logcat_tags
 
   # Desktop-only.
   shell_config.use_osmesa = script_args.use_osmesa
 
   if (shell_config.android and not shell_config.origin and
       inferred_paths['build_dir_path']):
     shell_config.origin = inferred_paths['build_dir_path']
 
@@ skipping 39 matching lines @@
 
     if 'content_handlers' in config:
       try:
         for (mime_type,
              content_handler_url) in config['content_handlers'].iteritems():
           shell_config.content_handlers[mime_type] = content_handler_url
       except (ValueError, KeyError):
         raise ShellConfigurationException('Failed to parse content_handlers in '
                                           'the config file.')
   return shell_config