Small fix to the extension test

[elliette]

Fixes the following error when running the extension test locally:

  Evaluation failed: TypeError: Cannot read properties of undefined (reading 'windowId')
      at __puppeteer_evaluation_script__:3:18
  package:puppeteer/src/page/execution_context.dart 169:11  ExecutionContext._evaluateInternal
  ===== asynchronous gap ===========================
  package:puppeteer/src/page/execution_context.dart 76:20   ExecutionContext.evaluate
  ===== asynchronous gap ===========================
  test/puppeteer/extension_test.dart 239:11                 _getWindowId
  ===== asynchronous gap ===========================
  test/puppeteer/extension_test.dart 167:30                 main.<fn>.<fn>.<fn>

[annagrin]

LGTM!