goma: use GOMA_SERVICE_ACCOUNT_JSON_FILE (reland #1)

scripts/slave/recipes/findit/chromium/test.expected/none_swarming_tests.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[build]\\scripts\\slave\\kill_processes.py"
     ],
     "cwd": "[SLAVE_BUILD]",
     "name": "taskkill"
   },
@@ skipping 540 matching lines @@
       "--src-dir",
       "[SLAVE_BUILD]\\src",
       "--goma-cache-dir",
       "[GOMA_CACHE]",
       "--build-tool",
       "ninja",
       "--compiler",
       "goma",
       "--goma-jsonstatus",
       "/path/to/tmp/json",
+      "--goma-service-account-json-file",
+      "C:\\creds\\service_accounts\\service-account-goma-client.json",
       "--goma-dir",
       "[CACHE]\\cipd\\goma",
       "--ninja-ensure-up-to-date",
       "--",
       "gl_tests"
     ],
     "cwd": "[SLAVE_BUILD]",
     "name": "test r1.compile",
     "~followup_annotations": [
       "@@@STEP_NEST_LEVEL@1@@@",
@@ skipping 80 matching lines @@
       "@@@STEP_LOG_END@report@@@",
       "@@@SET_BUILD_PROPERTY@report@{\"metadata\": {\"regression_range_size\": 1}, \"result\": {\"r1\": {\"gl_tests\": {\"failures\": [\"Test.One\", \"Test.Two\"], \"status\": \"failed\", \"valid\": true}}}}@@@"
     ]
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]