Add json test results format support for SwarmingIsolatedScriptTest

scripts/slave/recipes/chromium.expected/dynamic_swarmed_sharded_isolated_script_test_missing_shard.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_MODULE[depot_tools::bot_update]/resources/bot_update.py",
       "--spec",
       "cache_dir = '[GIT_CACHE]'\nsolutions = [{'deps_file': '.DEPS.git', 'managed': True, 'name': 'src', 'url': 'https://chromium.googlesource.com/chromium/src.git'}]",
       "--patch_root",
       "src",
@@ skipping 411 matching lines @@
     "name": "telemetry_gpu_unittests on Ubuntu-12.04",
     "~followup_annotations": [
       "step returned non-zero exit code: 1",
       "@@@STEP_LOG_LINE@outdir_json@{@@@",
       "@@@STEP_LOG_LINE@outdir_json@  \"0/output.json\": \"{\\\"successes\\\": [\\\"test1.Test1\\\", \\\"test2.Test2\\\"], \\\"failures\\\": [], \\\"valid\\\": true, \\\"times\\\": {\\\"test2.Test2\\\": 0.1, \\\"test1.Test1\\\": 0.1}}\", @@@",
       "@@@STEP_LOG_LINE@outdir_json@  \"summary.json\": \"{\\\"shards\\\": [{\\\"internal_failure\\\": false, \\\"failure\\\": false, \\\"exit_code\\\": \\\"0\\\"}, {\\\"internal_failure\\\": false, \\\"failure\\\": false, \\\"exit_code\\\": \\\"0\\\"}]}\"@@@",
       "@@@STEP_LOG_LINE@outdir_json@}@@@",
       "@@@STEP_LOG_END@outdir_json@@@",
       "@@@STEP_LOG_LINE@no_results_exc@no results from shard #1@@@",
       "@@@STEP_LOG_END@no_results_exc@@@",
-      "@@@STEP_LOG_LINE@invalid_results_exc@'failures'@@@",
+      "@@@STEP_LOG_LINE@invalid_results_exc@KeyError('valid',)@@@",
       "@@@STEP_LOG_END@invalid_results_exc@@@",
       "@@@STEP_LINK@shard #0@https://chromium-swarm.appspot.com/user/task/10000@@@",
       "@@@STEP_LINK@shard #1@https://chromium-swarm.appspot.com/user/task/10100@@@",
       "@@@STEP_FAILURE@@@"
     ]
   },
   {
     "name": "$result",
     "reason": "1 tests failed: ['telemetry_gpu_unittests']",
     "recipe_result": null,
     "status_code": 1
   }
 ]