Only set the ADB path if it exists.

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.
 """
 
@@ skipping 41 matching lines @@
 
   root_path = find_ancestor_with(out_build_dir)
   params = collections.defaultdict(lambda: None)
   if not root_path:
     return params
 
   build_dir_path = os.path.join(root_path, out_build_dir)
   params['build_dir_path'] = build_dir_path
   if is_android:
     params['shell_path'] = os.path.join(build_dir_path, 'apks', 'MojoShell.apk')
-    params['adb_path'] = os.path.join(root_path, 'third_party', 'android_tools',
-                                'sdk', 'platform-tools', 'adb')
+    adb_path = os.path.join(root_path, 'third_party', 'android_tools',
+                            'sdk', 'platform-tools', 'adb')
+    if os.path.isfile(adb_path):
+      params['adb_path'] = adb_path
   else:
     params['shell_path'] = os.path.join(build_dir_path, 'mojo_shell')
 
   mojo_version_file = find_within_ancestors('MOJO_VERSION')
   if mojo_version_file:
     with open(mojo_version_file) as f:
       params['mojo_version'] = f.read().strip()
   return params
 
 
@@ skipping 18 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