Check that device file exists first before reverting.

scripts/slave/recipe_modules/chromium_android/api.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.
 
 import contextlib
 import datetime
 import json
 import os
 import re
 import sys
@@ skipping 326 matching lines @@
       max_battery_temp=max_battery_temp,
       remove_system_webview=remove_system_webview)
 
   @property
   def blacklist_file(self):
     return self.out_path.join('bad_devices.json')
 
 
   def revert_device_file_format(self):
     # If current device file is jsonified, revert it back to original format.
-    with self.m.step.nest('fix_device_file_format'):
-      file_contents = self.m.file.read(
-          'read_device_file', self.known_devices_file,
-          test_data='device1\ndevice2\ndevice3')
-      try:
-        devices = json.loads(file_contents)
-        self.m.step.active_result.presentation.step_text += (
-            'file format is json, reverting')
-        old_format = '\n'.join(devices)
-        self.m.file.write(
-            'revert_device_file', self.known_devices_file, old_format)
-      except ValueError:
-        # File wasn't json, so no need to revert.
-        self.m.step.active_result.presentation.step_text += (
-            'file format is compatible')
+    if self.m.path.exists(self.known_devices_file):
+      with self.m.step.nest('fix_device_file_format'):
+        file_contents = self.m.file.read(
+            'read_device_file', self.known_devices_file,
+            test_data='device1\ndevice2\ndevice3')
+        try:
+          devices = json.loads(file_contents)
+          self.m.step.active_result.presentation.step_text += (
+              'file format is json, reverting')
+          old_format = '\n'.join(devices)
+          self.m.file.write(
+              'revert_device_file', self.known_devices_file, old_format)
+        except ValueError:
+          # File wasn't json, so no need to revert.
+          self.m.step.active_result.presentation.step_text += (
+              'file format is compatible')
 
   def device_status_check(self, restart_usb=False, **kwargs):
     # TODO(bpastene): Remove once chromium revisions prior to
     # crrev.com/1faecde0c03013b6cd725da413339c60223f8948 are no longer tested.
     # See crbug.com/619707 for context.
     self.revert_device_file_format()
 
     # TODO(phajdan.jr): Remove path['build'] usage, http://crbug.com/437264 .
     args = [
         '--adb-path', self.m.adb.adb_path(),
@@ skipping 987 matching lines @@
       script = self.c.test_runner
       if wrapper_script_suite_name:
         script = self.m.chromium.output_dir.join('bin', 'run_%s' %
                                                  wrapper_script_suite_name)
       else:
         env = kwargs.get('env', {})
         env['CHROMIUM_OUTPUT_DIR'] = env.get('CHROMIUM_OUTPUT_DIR',
                                              self.m.chromium.output_dir)
         kwargs['env'] = env
       return self.m.python(step_name, script, args, **kwargs)