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

This prevents an AttributeError in AndroidShell when no adb_path is specified on the command line.

Fixes domokit/devtools#33

R=ppi@chromium.org, ppi

Review URL: https://codereview.chromium.org/1323973002 .

Cr-Mirrored-From: https://github.com/domokit/mojo
Cr-Mirrored-Commit: a6b862ed82a1a1d34d1f073e942e6a6964baf696

devtoolslib/shell_config.py

diff --git a/devtoolslib/shell_config.py b/devtoolslib/shell_config.py
index 6a93cb0..b7f59d2 100644
--- a/devtoolslib/shell_config.py
+++ b/devtoolslib/shell_config.py
@@ -152,7 +152,8 @@
   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