goma: use GOMA_SERVICE_ACCOUNT_JSON_FILE (reland #1)

scripts/slave/recipes/chromium.expected/full_chromium_webkit_WebKit_Win_x64_Builder.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[build]\\scripts\\slave\\kill_processes.py"
     ],
     "cwd": "[SLAVE_BUILD]",
     "name": "taskkill"
   },
@@ skipping 301 matching lines @@
       "--goma-cache-dir",
       "[GOMA_CACHE]",
       "--build-tool",
       "ninja",
       "--build-data-dir",
       "[ROOT]\\build_data_dir",
       "--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",
       "--",
       "blink_tests",
       "browser_tests",
       "browser_tests_run"
     ],
     "cwd": "[SLAVE_BUILD]",
     "name": "compile",
     "~followup_annotations": [
@@ skipping 102 matching lines @@
     ],
     "cwd": "[SLAVE_BUILD]",
     "name": "process_dumps"
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]