goma: back to build-provided client and disable oauth2

scripts/slave/recipes/dart/dartium_build.expected/basic_mac_64_MirrorFalse.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[depot_tools]/gclient.py",
       "config",
       "--spec",
       "cache_dir = '[GIT_CACHE]'\nsolutions = [{'deps_file': 'DEPS', 'managed': True, 'name': 'dartium.deps', 'url': 'https://dart.googlecode.com/svn/branches/bleeding_edge/deps/dartium.deps'}]"
     ],
@@ skipping 110 matching lines @@
       "--src-dir",
       "[SLAVE_BUILD]/src",
       "--goma-cache-dir",
       "[GOMA_CACHE]",
       "--build-tool",
       "ninja",
       "--compiler",
       "goma-clang",
       "--goma-jsonstatus",
       "/path/to/tmp/json",
-      "--goma-service-account-json-file",
-      "/creds/service_accounts/service-account-goma-client.json",
       "--goma-dir",
       "[BUILD]/goma",
       "--",
       "All",
       "chromium_builder_tests"
     ],
     "cwd": "[SLAVE_BUILD]",
     "name": "compile",
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@json.output (invalid)@null@@@",
       "@@@STEP_LOG_END@json.output (invalid)@@@"
     ]
   },
   {
     "cmd": [
       "python",
       "-u",
       "[SLAVE_BUILD]/src/dart/tools/dartium/multivm_archive.py",
       "12345"
     ],
     "cwd": "[SLAVE_BUILD]",
     "name": "archive_build"
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]