devtools: download shell binaries and set origin when run w/ --mojo-version.

mojo/devtools/common/devtoolslib/paths.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.
 
 """Supports inferring locations of files in default checkout layouts.
 
 These functions allow devtools scripts to work out-of-the-box with regular Mojo
 checkouts.
 """
 
 import collections
 import os.path
 import sys
 
+DEVTOOLS_ROOT = os.path.join(os.path.dirname(os.path.realpath(__file__)), '..')
+
 
 def find_ancestor_with(relpath, start_path=None):
   """Returns the lowest ancestor of this file that contains |relpath|."""
   cur_dir_path = start_path or os.path.abspath(os.path.dirname(__file__))
   while True:
     if os.path.exists(os.path.join(cur_dir_path, relpath)):
       return cur_dir_path
 
     next_dir_path = os.path.dirname(cur_dir_path)
     if next_dir_path != cur_dir_path:
@@ skipping 69 matching lines @@
   # Rare case, it's not even in PATH, look upward up to root.
   root_dir = os.path.dirname(os.path.abspath(__file__))
   previous_dir = os.path.abspath(__file__)
   while root_dir and root_dir != previous_dir:
     i = os.path.join(root_dir, 'depot_tools')
     if _is_real_depot_tools(i):
       return i
     previous_dir = root_dir
     root_dir = os.path.dirname(root_dir)
   return None