goma: use GOMA_SERVICE_ACCOUNT_JSON_FILE (reland #1)

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

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_MODULE[depot_tools::bot_update]/resources/bot_update.py",
       "--master",
       "chromium.webkit",
       "--builder",
       "WebKit Linux",
@@ skipping 261 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",
+      "/creds/service_accounts/service-account-goma-client.json",
       "--goma-dir",
       "[CACHE]/cipd/goma",
       "--ninja-ensure-up-to-date",
       "--",
       "blink_tests"
     ],
     "cwd": "[SLAVE_BUILD]",
     "name": "compile",
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@json.output (invalid)@null@@@",
@@ skipping 63 matching lines @@
       "@@@STEP_LINK@layout_test_results@https://storage.googleapis.com/chromium-layout-test-archives/WebKit_Linux/571/layout-test-results/results.html@@@",
       "@@@STEP_LINK@(zip)@https://storage.googleapis.com/chromium-layout-test-archives/WebKit_Linux/571/layout-test-results.zip@@@"
     ]
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]