Check that device file exists first before reverting.

scripts/slave/recipe_modules/chromium_android/example.py

 # Copyright 2014 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.
 
 from recipe_engine.types import freeze
 
 DEPS = [
     'adb',
     'chromium',
     'chromium_android',
@@ skipping 363 matching lines @@
              stream='stdout')) +
          api.override_step_data('Read test result and report failures',
              api.raw_io.output(result_xml_with_unexpected_failure_method)))
 
   yield (api.test('webview_cts_invalid_output') +
          properties_for('webview_cts') +
          api.override_step_data('Run CTS', api.raw_io.stream_output(
              'Invalid CTS output here...',
              stream='stdout')))
 
+  yield (api.test('device_file_match') +
+         properties_for('tester') +
+         api.path.exists(
+             api.path['build'].join('site_config', '.known_devices')))
+
   yield (api.test('device_file_format_mismatch') +
          properties_for('tester') +
+         api.path.exists(
+             api.path['build'].join('site_config', '.known_devices')) +
          api.override_step_data('fix_device_file_format.read_device_file', 
              api.raw_io.output('["device1", "device2"]')))
 
   yield (api.test('tombstones_m53') +
          properties_for('tester') +
          api.override_step_data('get version (2)',
              api.raw_io.output('MAJOR=53\nMINOR=0\nBUILD=2800\nPATCH=0\n')))