Update from https://crrev.com/304121

build/android/pylib/device/adb_wrapper.py

 # Copyright 2013 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.
 
 """This module wraps Android's adb tool.
 
 This is a thin wrapper around the adb interface. Any additional complexity
 should be delegated to a higher level (ex. DeviceUtils).
 """
 
@@ skipping 134 matching lines @@
 
     Args:
       remote: Path on the device filesystem.
       local: Path on the host filesystem.
       timeout: (optional) Timeout per try in seconds.
       retries: (optional) Number of retries to attempt.
     """
     self._DeviceAdbCmd(['pull', remote, local], timeout, retries)
     _VerifyLocalFileExists(local)
 
-  def Shell(self, command, expect_rc=None, timeout=_DEFAULT_TIMEOUT,
+  def Shell(self, command, expect_rc=0, timeout=_DEFAULT_TIMEOUT,
             retries=_DEFAULT_RETRIES):
     """Runs a shell command on the device.
 
     Args:
       command: The shell command to run.
-      expect_rc: (optional) If set checks that the command's return code matches
-        this value.
+      expect_rc: (optional) Check that the command's return code matches this
+        value. Default is 0. If set to None the test is skipped.
       timeout: (optional) Timeout per try in seconds.
       retries: (optional) Number of retries to attempt.
 
     Returns:
       The output of the shell command as a string.
 
     Raises:
       device_errors.AdbCommandFailedError: If the return code doesn't match
         |expect_rc|.
     """
@@ skipping 216 matching lines @@
     """Restarts the adbd daemon with root permissions, if possible.
 
     Args:
       timeout: (optional) Timeout per try in seconds.
       retries: (optional) Number of retries to attempt.
     """
     output = self._DeviceAdbCmd(['root'], timeout, retries)
     if 'cannot' in output:
       raise device_errors.AdbCommandFailedError(['root'], output)