Check that device file exists first before reverting.

scripts/slave/recipe_modules/chromium_android/example.expected/downgrade_install_tester_basic.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "\nimport os, sys\n\nbuild_path = sys.argv[1]\nif os.path.exists(build_path):\n  for (path, dir, files) in os.walk(build_path):\n    for cur_file in files:\n      if cur_file.endswith('index.lock'):\n        path_to_file = os.path.join(path, cur_file)\n        print 'deleting %s' % path_to_file\n        os.remove(path_to_file)\n",
       "[SLAVE_BUILD]"
     ],
     "name": "cleanup index.lock",
     "~followup_annotations": [
@@ skipping 251 matching lines @@
       "--adb-path",
       "[SLAVE_BUILD]/src/third_party/android_tools/sdk/platform-tools/adb"
     ],
     "env": {
       "GOMA_SERVICE_ACCOUNT_JSON_FILE": "/creds/service_accounts/service-account-goma-client.json",
       "PATH": "[SLAVE_BUILD]/src/third_party/android_tools/sdk/platform-tools:[SLAVE_BUILD]/src/build/android:%(PATH)s"
     },
     "name": "authorize_adb_devices"
   },
   {
-    "cmd": [],
-    "name": "fix_device_file_format"
-  },
-  {
-    "cmd": [
-      "python",
-      "-u",
-      "\nimport shutil\nimport sys\nshutil.copy(sys.argv[1], sys.argv[2])\n",
-      "[BUILD]/site_config/.known_devices",
-      "/path/to/tmp/"
-    ],
-    "name": "fix_device_file_format.read_device_file",
-    "~followup_annotations": [
-      "@@@STEP_NEST_LEVEL@1@@@",
-      "@@@STEP_TEXT@file format is compatible@@@"
-    ]
-  },
-  {
     "cmd": [
       "[SLAVE_BUILD]/src/build/android/buildbot/bb_device_status_check.py",
       "--adb-path",
       "[SLAVE_BUILD]/src/third_party/android_tools/sdk/platform-tools/adb",
       "--blacklist-file",
       "[SLAVE_BUILD]/src/out/bad_devices.json",
       "--json-output",
       "/path/to/tmp/json",
       "--known-devices-file",
       "[BUILD]/site_config/.known_devices"
@@ skipping 112 matching lines @@
       "-t",
       "Release"
     ],
     "env": {
       "GOMA_SERVICE_ACCOUNT_JSON_FILE": "/creds/service_accounts/service-account-goma-client.json",
       "PATH": "[SLAVE_BUILD]/src/third_party/android_tools/sdk/platform-tools:[SLAVE_BUILD]/src/build/android:%(PATH)s"
     },
     "name": "provision_devices"
   },
   {
-    "cmd": [],
-    "name": "fix_device_file_format (2)"
-  },
-  {
-    "cmd": [
-      "python",
-      "-u",
-      "\nimport shutil\nimport sys\nshutil.copy(sys.argv[1], sys.argv[2])\n",
-      "[BUILD]/site_config/.known_devices",
-      "/path/to/tmp/"
-    ],
-    "name": "fix_device_file_format.read_device_file (2)",
-    "~followup_annotations": [
-      "@@@STEP_NEST_LEVEL@1@@@",
-      "@@@STEP_TEXT@file format is compatible@@@"
-    ]
-  },
-  {
     "cmd": [
       "[SLAVE_BUILD]/src/build/android/buildbot/bb_device_status_check.py",
       "--adb-path",
       "[SLAVE_BUILD]/src/third_party/android_tools/sdk/platform-tools/adb",
       "--blacklist-file",
       "[SLAVE_BUILD]/src/out/bad_devices.json",
       "--json-output",
       "/path/to/tmp/json",
       "--known-devices-file",
       "[BUILD]/site_config/.known_devices"
@@ skipping 304 matching lines @@
       "PATH": "[SLAVE_BUILD]/src/third_party/android_tools/sdk/platform-tools:[SLAVE_BUILD]/src/build/android:%(PATH)s"
     },
     "name": "stack_tool_for_asan"
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]