Check that device file exists first before reverting.

scripts/slave/recipe_modules/chromium_android/example.py

Index: scripts/slave/recipe_modules/chromium_android/example.py
diff --git a/scripts/slave/recipe_modules/chromium_android/example.py b/scripts/slave/recipe_modules/chromium_android/example.py
index c49bae08d6c334cff7d03627b37deb900f6ed717..42185c6825a28dbf7b093a6912fbe32918b951c8 100644
--- a/scripts/slave/recipe_modules/chromium_android/example.py
+++ b/scripts/slave/recipe_modules/chromium_android/example.py
@@ -381,8 +381,15 @@ def GenTests(api):
              '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"]')))