Do not assume that perf tests always have end_time

scripts/slave/recipe_modules/chromium_android/example.expected/perf_tests_failure.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 785 matching lines @@
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@json.output@[@@@",
       "@@@STEP_LOG_LINE@json.output@  {@@@",
       "@@@STEP_LOG_LINE@json.output@    \"device_affinity\": 0, @@@",
       "@@@STEP_LOG_LINE@json.output@    \"end_time\": 1443438432.949711, @@@",
       "@@@STEP_LOG_LINE@json.output@    \"has_archive\": true, @@@",
       "@@@STEP_LOG_LINE@json.output@    \"test\": \"perf_test.foo\"@@@",
       "@@@STEP_LOG_LINE@json.output@  }, @@@",
       "@@@STEP_LOG_LINE@json.output@  {@@@",
       "@@@STEP_LOG_LINE@json.output@    \"device_affinity\": 0, @@@",
-      "@@@STEP_LOG_LINE@json.output@    \"end_time\": 1470050195.193213, @@@",
       "@@@STEP_LOG_LINE@json.output@    \"test\": \"page_cycler.foo\"@@@",
       "@@@STEP_LOG_LINE@json.output@  }@@@",
       "@@@STEP_LOG_LINE@json.output@]@@@",
       "@@@STEP_LOG_END@json.output@@@"
     ]
   },
   {
     "allow_subannotations": true,
     "cmd": [
       "python",
@@ skipping 219 matching lines @@
     },
     "name": "stack_tool_for_asan"
   },
   {
     "name": "$result",
     "reason": "sharded perf tests failed [StepFailure(\"Step('perf_test.foo') failed with return_code 1\",)]",
     "recipe_result": null,
     "status_code": 1
   }
 ]